The folks at Worldchanging point out the critical flaws in Walkscore. I had a similar take on the site about a year ago, though one that was a lot less in depth. Check out the Worldchanging article for a very insightful take on why WalkScore’s approach is outdated (terrible business model), as well as some commentary on the Second Life tool called Carbon Goggles.
My take on Carbon Goggles is something between “obvious” and “pointless.” Second Life has utterly failed to impress me in the suspension-of-disbelief department, thus I’m much more likely to be moved by data on carbon impact than on something that gimmicky. I suspect that while such gimmicks do tend to be effective in getting points across to folks who are not intimately familiar with the subject at hand, the Second Life audience is one that is not ill-educated on climate change.