I strongly suspect that they do. I think there have even been some published articles about various govt. agencies developing (or looking to develop) technology for this kind of manipulation. I don't have a citation handy, but if I can scare up one of the articles I think I'm remembering, I'll post it.

Edit: Here you go, they they definitely have the technology, but - as you might expect - they claim it isn't used on Americans. But in light of everything else we've learned the past few weeks, would anybody be surprised to find out that they are, indeed, using this on Americans?

http://www.guardian.co.uk/technology/2011/mar/17/us-spy-oper...