Hey I poo pood this for years in my educational presentations.
Turns out- they do change colors to blend in, in response to predators. Not only that, but they change in ways that specific predators will find them difficult to see (change for a bird vs a snake).
Somewhere in my posts in the last year or two was a link to a scientific paper on the study that showed this.
I'm too lazy to look for the paper link, but I search engined it (ixquick) and this nice link popped up:
http://www.newscientist.com/article/dn13944-chameleons-finetune-camouflage-to-predators-vision.html
So nowadays when I talk about color change and chameleons, it is a little more complex than thermoregulation and communication.
Edit- Oh here's a link to the paper I think:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2610148/
Edit edit- so I guess we have seen yet another one here on the forums, the myth that it is a myth that chameleons change colors to blend in. LOL