I’ve been explaining the value of IMAP email to clients for years. It’s far superior to POP, especially when you check your email from more than one computer or mobile device. Plus, if your computer crashes and you have no other backups, at least your email is safe! I set up pretty much all of my clients with it.
Now, Apple and MobileMe have done a good job explaining it too. In this article, they explain why your MobileMe account uses IMAP over POP. It’s worth a read… and if you’re still using POP, think seriously about switching!