My wife's mobile me account does the same thing on her new macbook. Just signed her up last week and has been constantly saying password rejected but when i enter the password email works fine for a while (sending/receiving all work ok) then the message pops back up again!!! Tried changing password, uninstall and install again but still the message appears. Idisk works fine as does mobile me syncing. I have the same set up on my imac and i have had no problems!!! Cleared keychain also but still have not been able to solve it. 🙁