MobileMe not working in Mail!
If MobileMe mail doesn't work the way it should in Mail, you can change the port used for the connection and it usually solves everything. This is actually AppleCare's recommendation. Checking the box next to "Use SSL" in the account preferences will change it from the default 143 to 993. I have had this problem happen a few times with new Macs and using this has always solved the problem.