Eddie - I don't know if you got this sorted out yet, but I just became a MobileMe user, and initially had some mail issues.
I went to the support site, and started a live chat, and actually got the MobileMe mail to work with their help. Frankly, I think the mail feature may be a bit dodgy. But I've only had it set up for one day.
Be sure you are logged into MobileMe by checking it in your system preferences. If you are, then go into mail -> Accounts.
If you have a Mobileme account already set up (and know your settings) try deleting it, then try creating it again. You may get the correct setup, i.e. MobileMe IMAP. If not - try the live chat. Worked for me! Good Luck!
It's a big universe, no one can know it all!
.... Si hoc legere scis, nimis eruditionis habes!
DP 1.8 G5 w 2Gb RAM, 250 and 160 Gb drives, MacBook Pro 2.0, 2Gb RAM, 160Gb drive