Your iPhone MobileMe account randomly and spontaneously changing from [username]@me.com to [username]@mac.com? You're not alone.
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.
The only known way to "fix" it at this point is to delete and re-add your MobileMe account as .me, and then keep your fingers crossed it doesn't change again.
That, and hope Apple fixes it with iPhone 3.0.x or 3.1.