Google suggest changing back to @googlemail.com, waiting two days and restarting the phone, but that hasn't worked either.
Anyone had any luck with this?
I had this issue with my old G1. The only resolution I was able to find was to do a factory reset on the phone and re-enter the Google ID with the @gmail.com extension, as OS version I was running does not have the ability to remove your Google account.
Newer OS versions do seem to have the ability to remove your Google account from the phone and re-enter it with the new extension as I've been able to do this on Froyo at least.