If you are having problems with your iPhone MobileMe account changing from @me.com to @mac.com you are not on your own.
Just like the missing app icon bug, this is unfortunately another one of those issues that also affected some betas, and some would say continues to be a problem now that the iPhone 3.0 has gone into release. Sadly, once the little bug manifests itself, MobileMe wont allow you to edit it back, and once everything changes from .me to .mac it can be more than a little annoying.
Okay, so now for the fix, the only way known at present is to delete and then re add your MobileMe account as .me, and then just hope and keep everything crossed that it doesn’t change back again. Hopefully Apple will have this fixed with iPhone 3.0x or 3.1.
Source – theiphoneblog