You can easily do it with the mail.app. It's very easy with Lion, you just need to add a gmail account* and then add a mobileme account. All you have to give is your login credentials and mail.app will do the rest. You will then see 2 accounts in mail, 1 for gmail and 1 for mobilme.
All you then need to do is copy your folders overs from gmail to mobilme.
* You might have to first enable IMAP under gmail settings -> Forwarding & Pop/IMAP