Okay, while this is slightly off topic, I figured this was the best thread for it, that I could see...
I, too, tried changing my e-mail in iTunes to my MM e-mail address, but get the same error. And then I go to myinfo.apple.com to see if I can find what's wrong, and it shows that my MM username and password don't exist or are incorrect, and I know they're correct.
So, is this Apple ID different from my MobileMe account? Is this why I also can't sync my damn MacBook to MobileMe? This is just so damn frustrating and I can't even use the service correctly/how it's supposed to be working, and it feels like a waste of money... Any suggestions?