A few weeks ago we pointed out a new service msgpush.com which would do push updates from an IMAP server to your iPhone.
Since then we've been told about another service that does this as well, http://nuevasync.com. For email push, NuevaSync isn't free, but their system integrates slightly better.
msgpush.com creates a separate "notification" mail account on your phone, which gets the push notifications, but you have to leave your regular IMAP account configured to read the email contents. NuevaSync creates a single account on the phone that completely integrates the 2-way IMAP \<-> phone push and synchronisation.
NuevaSync also support a number of other phones, such as some Windows Mobile and Nokia phones. See their FAQ for details.
We’re interested in hearing about user experiences of the service in our forum.