I'm having this problem too. My iPhone seems to work fine. When texting a known iOS5 device, it says "iMessage" in the text line and the text balloons are blue.
I just update my wife's iPhone, and she can iMessage my phone; but I can't iMessage her (it defaults to green text message from my phone). Basically her phone number is not recognized as belonging to an iPhone. I can iMessage her, however, when I text to her Apple ID.
Any suggestions would be appreciated! Am hoping it has something to do with AT&T lag and slow servers today.