When I signed up for a MobileMe account, I used a different password than my Apple account. This seemed to have confused my iPhone mightily, and I had this experience when trying to enter my password in order to download updates from the App Store:

Ultimately I went in and changed my password on Apple.com and that allowed things to proceed.

Of additional frustration was the inability to paste into the password field – something that is very useful when you use hashed passwords.