Google has just rolled out free IMAP access for Gmail, and it will reach all users within a few days:

Without IMAP, it was difficult to keep all your devices — iPhone, your desktop client, to your webmail — synced up. IMAP lets you sync your Gmail inbox across devices, and whether you read or write your email on your phone or on your desktop, changes you make to Gmail will be seen from anywhere you access your inbox.

More here: http://mail.google.com/mail/help/about_whatsnew.html