Our sun turns 4 billion kilograms of matter into energy every second- refining the proper isotopes on that scale, transporting them in that quantity to supply a massive reactor would be a lot more difficult- I would assume- than a giant solar array...

Personally, I think in a year or two the answer to what we are seeing is interesting, but not THAT interesting- the 'megastructures' speculation will be remembered as a cautionary tale much like the canals on mars: http://astroengine.com/2009/04/24/in-1911-martians-were-building-canals/

I mainly posted this because it is a sneak peek at how the actual discovery of ETI would most likely unfold... Anomalous data leads to a closer look, and follow up observations over the course of years slowly rule out everything natural... much to the consternation of the conspiracy nuts the whole saga plays out quite publicly....

I once posted a short story here in an earlier incarnation of this board. It was about the discovery process for Type III Kardashev civilization.... (Galaxies are found that seem to be mostly dark matter... further study shows they are actually galaxies where a large fraction of the stars have solar collectors around them)

Yes we are making the assumption that some ETI would have the human-like desire to engineer on a grand scale, however the ONLY way we will discover ETI is if they do have this drive.

ETI that lives happily within the means of their planet will almost certainly never be detectable to us... we will only find ETI for which one planet, or one solar system is not enough-

Perhaps that should worry us... 8^)