Rui Carmo...for he is to be rightfully worshipped, having just released a Mail.app plugin that enables IMAP IDLE, one of my age-old gripes with it.
It works by opening secondary IMAP connections to your server(s) and mule-kicking Mail.app to wakefulness when new messages arrive (rather than fixing Mail.app itself), but it will do for now.
So, what's taking Apple so long, then?
(via Tim Gaden's Hawk Wings)