Google today announced IMAP support for its immensely popular GMail email service.
Previously, GMail users have either had to connect using POP3 in Apple's Mail client or via Google's mobile web interface. However, POP3 has notable limitations when mail may be accessed from more than one client (i.e. a web client and a desktop mail client). In such situations, IMAP is much more suited to the task.
Interestingly, Google appears to be very much aware that some of the people most interested in the new functionality will be iPhone users. They have posted the following video tutorial for how to set up GMail on the iPhone:
The capability of course does not apply only to the iPhone, as users of desktop-based mail clients could use IMAP as well using the same settings.
Additional Reading: IMAP vs. POP (via Wikipedia)