Don't be tempted to change your Google account from an @googlemail.com account to an @gmail.com account even if google advise you to. If you do, it will break your accounts on your phone like Talk, Market and so on. It is likely that you may have to factory-reset your phone to fix this.
At the moment, the resolution for this is a factory reset of your phone or restoring your account back to @googlemail.com, so be warned before modifying your account!
The way to reset your account seems to be to:
- Go to your Google account setting on your PC (not your phone)
- Click on Accounts and Import
- Under 'Send Mail As' click on the 'switch to googlemail.com'
- Job done.