Your iPhone MobileMe account randomly and spontaneously changing from [username]@me.com to [username]@mac.com? You're not alone.
Like the missing/mismatched app icon bug, this is another issue that was a problem to some during the betas, and continues to be a problem now that iPhone 3.0 has gone into release.
Unfortunately, once the bug manifests, MobileMe doesn't allow you to simply edit it back, and since everything changes from .me to .mac (your email address, SMTP server, etc.), it can be more than a little annoying.