Thanks to Marcia for pointing me towards some very interesting commentary.

Marcia's original link discussed Sedna and it's status as a planet

Brown's list of potential solar system planets based on the roundness criterion

Brown's Commentary on the proposed IAU definition of "planet"
"How am I going to vote on the IAU resolution? This one is easy to answer. I am not an IAU member, I took no part in drafting the resolution, and I get no vote. If I were to vote, however, I would have to decide that while the definition itself is viable the extra non-scientific beauracratic barrage attached to the resolution would doom it for me."
Note: he admits his objection is not based on science. My view is we don't expect our kids to name all the stars and if there are too many planets we shouldn't expect kids to name them all either. It's not a basis for scientific definition.

Brown's New York Times op-ed piece

In a fairly lengthy and more technical paper Planetesimals to Brown Dwarfs: What is a Planet? Brown very clearly discusses the continuity issue I have brought up many times from planetisimal to star. He strongly supports the need for significant transitions to mark boundaries as I do. Yet in all this discussion he fails to notice that only when it comes to planets does he start talking about limits on numbers. We don't do this for stars. We don't do this for brown dwarfs. We don't even do this for the upper size limit on Planets. Why are we doing it at the low end of the definition for planets?

Brown discusses a number of significant transitions above the roundness transition that might be used to mark transition to planetary status and I think they are good potential markers for a transition from dwarf planet to terrestrial planet status. Problem is I think these are harder boundaries to observe and validate and therefore should not mark as big a transition as from planetisimal to planet.

Some key comments from Brown:

"Nature, of course, does not worry about definitions. Objects are made in various contexts, with a continuum of masses, and using a number of formation mechanisms. It is humans who wish to apply labels, and to distinguish between “different” objects. We must accept the fact that, in placing boundaries on a continuum, there might be transitional objects which are quite similar to each other in many respects, yet which are defined to lie on opposite sides of our imposed (and hopefully not too arbitrary) lines. A scientific definition would find natural physical conditions for such boundaries. A successful definition can only be arrived at through a consensus process. Our purpose here is to lay out many of the arguments and considerations which go into it. Whether some “official” definition is ever codified (or some consensus definition comes into wide use) depends on both scientists and the lay public having reached some common understanding."

"From a physical point of view, the limiting objects (which everyone agrees are NOT planets) differ in two different ways from planets near those limits. At the lower end, the size and shape of rocks are quite different from those of small planets (while the composition and luminous mechanisms can be similar). At the upper end, the amount and mechanism of the luminosity of stars is somewhat different from those of large planets (while the composition and size may be similar). It therefore seems sensible to concentrate on the physical consequences of pressure support versus gravity at the lower end, and the luminosity mechanisms at the upper end (while composition should be left out in both cases). Of course, the luminosity of stars is also a result of the competition between pressure support and gravity, so we are being consistent in that sense. There are other physical transitions which depend on mass that we can also consider."

"While becoming spherical is perhaps the most obvious outward sign of increasing mass, the interior of the body begins to undergo many interesting geophysical transitions as the mass increases. An important transition from an object which is geophysically dead to one that is active is the transition to bodies large enough to have convection in their interiors. The threshold for convection scales as the Rayleigh number in the interior which scales roughly as D5 times the thermal contrast.This itself is likely to be an increasing function of D and a decreasing function of age –(for bodies of similar composition, where D is the diameter of the body). With such a strong dependence on planetary radius, solid state convection quickly becomes important somewhere in the size approximately between the Moon (D=3500 km) and Mars (D=6500 km). At a similar mass – a diameter of around 3000 km for a terrestrial body – the average gravitational energy per atom exceeds 1~eV, which is the typical energy of chemical reactions. A body this size has sufficient gravitational energy to substantially modify the initial chemical composition of its initial starting materials."

"One final threshold that occurs near this size range is that when the central pressure of the object begins to be large enough that the materials have significantly higher densities than their non-compressed forms. The central pressure at which such compression occurs is approximately equal to the bulk modulus of the constituent material, which is approximately 1 Mbar for rocky materials and 10 kbar for icy materials. If we assume hydrostatic equilibrium and incompressible materials, the central pressure in a body is P=(1.4 kbars)(ρ/1 g/cc)2 (R/1000 km) 2. Pressures high enough for significant compression are reached for a size of approximately 6000 km for rocky materials and only 1000 km for ice-rock mixtures. The true sizes needed for compression will be somewhat smaller, as the compression itself violates our incompressible assumption. The estimates of the body size for which these important transitions are reached are extremely rough and can be greatly influenced by composition, temperature, state of differentiation, and other details about the body. Nonetheless each of these transitions is conceptually well-defined and could equally well form the potential basis for drawing boundaries. After passing this mass range, there is not another significant qualitative change in the relation between pressure and gravity until masses greater than Jupiter. There was thought to be a significant “characteristics” boundary at about 10-15 earth masses, in the sense we didn’t think we would find “terrestrial” planets with larger masses. This is because the supply of heavy elements in the protoplanetary disk is only something like one percent that of hydrogen and helium (which we will henceforth refer to as “gas”). It is hard to gather a more massive object without also gathering a lot of gas. The planets Uranus and Neptune are transitional objects in this sense. Very recently, however, the discovery of an exoplanet (Sato et al. 2005, Fig. 2) with nearly the mass of Saturn but only two-thirds its size has thrown this theoretical expectation into doubt (it may have a rock/ice core of 60 Earth masses or more)."

After reading Browns comments I'd like to see a planet definition along the following lines:

Planet: A solid object orbiting a star with enough mass for gravity to force the object into a round shape.

Dwarf Planet: A planet too small to either initiate mantle convection or geochemical evolution.
Subclasses:
Ice dwarf: A dwarf planet with a density less than X indicating physical processes dominated by ice (ie
Pluto, Xena, and possibly Sedna, Quoar, and Varuna).
Rocky dwarf: A dwarf planet with a density greater than X indicating physical processes dominated by rock (
ie Ceres, and possibly Mercury and Vesta).

Note: Not sure where X should be, but I'm sure planetary scientists should have an idea and X should be a
maleable goal post based on future observations. My primary reason for this division is that ice dwarfs would
form at significantly smaller sizes than rocky dwarfs. X should be defined where there is a noticable
inflection point between properties proportional to density.

Terrestrial planet: A planet massive enough to initiate active mantle convection or geochemical evolution at some point in it's history yet too small to retain significant amounts of molecular hydrogen in it's atmosphere over geological time scales.

Notes: Volcanoes might be a simple clear cut marker. Mercury might be a dwarf rather than a terrestrial planet based on Brown's comments and these definitions.

Gas giant: A planet massive enough to retain significant amounts of molecular hydrogen in it's atmosphere over geological time scales yet too small to be classed as a brown dwarf (conveniently sidestepping that definition).

Brown's comments suggest additional divisions are possible and some astronomers have made a distinction between the blue twins and the striped giants in our own solar system.

Optional Double planet clause/exclusion:

Double planets allowed: Original proposal based on Barycenter.
Double planets excluded: Only the most massive object in a group of objects sharing an orbit around the Sun will be classified as a planet, the rest will be considered moons or satellites.

Regards,
Mark