IMAP, aox, 3G
A little bit of 3G first: A 3G connection is in one of several modes, ranging from PCH (which uses hardly any power and can't transfer payload data) to DCH (which uses much power and is used for bulk transfers).
The way Archiveopteryx handles IMAP, POP and SMTP is very battery-friendly. Archiveopteryx permits the 3G device to stay in PCH mode almost always, and only forces DCH mode when the user requests a bulk transfer.
Specifically, Archiveopteryx tends to be curt when it talks, to talk seldom, avoid keepalives, and to use much longer timeouts than the standards require.
As a side remark, some IMAP clients may be considerably more battery-friendly than ohers. Using notify is better than idle (but almost never possible) and idle is in turn better than polling. Using bodystructure is better than body.header etc. Using compress helps. Using condstore helps. Adding all this up, the iphone mail.app (4.x) scores well, the android client not so well (2.1), don't know about Nokia's client, and c-client-based mail readers suck (film at 11).