A key problem is the temperatures. In the LPSC report by the TES scientists I cited, they note the temperature in the image they referenced was only around 141K when the spots were there, but when the temperature rose to around 265K the spots disappeared. This would favor the hypothesis of CO2 evaporating to reveal underlying dark areas.
However, in that case I'm puzzled why they still allow it might be due to water ice evaporating.
I've been informed by one of the authors of that report that in the image I cited the temperatures would likely also be around only 141K since it takes place in late winter. However, I would still like to see TES readings of the location. Actually it would probably be good to see TES readings of all the gully sites, since MGS images usually take place around 2pm local time when the temperatures would be near maximum and it is known temperatures at these latitudes can exceed 0 degrees C.

TES only has a resolution of 3km, too large for spots of this size. Perhaps THEMIS could resolve them.
There could be large communities of these organisms that bloom at the same time, like algae bloom on ocean surfaces, to make them visible at meter scale imagery.
I agree that at this resolution scale these images would not be enough to resolve the issue if they are biogenic or not. Perhaps the 2005 orbiter imagery at the 8" scale would be.

A report at the 5th International Conference on Mars mentioned a tentative identification of methane on Mars:

Title: High-Resolution Spectroscopy of Mars: Recent Results and Implications for Atmospheric Evolution
Authors: Krasnopolsky, V. A.; Owen, T. C.; Maillard, J. P.
Journal: The Fifth International Conference on Mars, July 19-24, 1999, Pasadena, California, abstract no. 6016
http://adsabs.harvard.edu/cgi-bin/bib_query?1999ficm.conf.6016K

However, I haven't heard anything else about this since then. On the other hand, this report also discussed the authors attempt to make the first detection of molecular hydrogen on Mars using the FUSE spacecraft. This detection has just been announced in this weeks Science. So perhaps an announcement of the detection of methane is also in the offing?

Bob Clark