Left field thought... you've not just changed from @googlemail to @gmail have you?
This caused problems with people signing into the android market. It keeps asking me at the top of my Gmail page if I want to change but I'm keeping it as it was cos I don't want to upset the apple cart on the phone.
If you have you need to clear your data in all the gmail apps in the Manage Applications part of the Settings Menu. Word of caution, make sure your contacts are saved to SD card because I'm sure if you are using Google Sync and not SE sync you lose them too. You may lose your apps from market too, but would think that's just a matter of reinstalling. Sorry if I sound vague, I haven't done it so going from memory of what I read elsewhere. Have a look at threads to do with Android Market not allowing sign in or password incorrect.