It appears as though Google has decided to enable IMAP for some Gmail (and Google Apps for Your Domain) accounts. As it happens, all of my Google Apps for Your Domain accounts now have IMAP enabled, so I’m playing around with the settings and changes now.
IMAP on Gmail is a great thing because it means that you can setup Gmail in several mail clients (Thunderbird, Windows Mobile mail, etc) and they will all remain in sync – no more reading the same message twice or manually having to mark things as read. Once again, thanks Google!
For more info on IMAP (history etc) see this wikipedia article.