Unbelievably, I still get to learn new things every day. Today, I discovered that my favorite desktop mail client --- Thunderbird --- does IMAP IDLE. Email flashes on screen the instant it's received on the IMAP server, and no wasted cycles polling the server endlessly.
This is pretty humbling considering that we've been using Thunderbird as our e-mail client of choice in the office for a while, and the IMAP protocol for over 10 years to fetch and manage our mail. We also have all our iPhones, E71s, Blackberries et al. doing IMAP (only the Nokia E Series and the G1 do IMAP IDLE correctly, AFAIK - Winmo - I'm not so sure. Apple tries to lock you into using MobileME by breaking IMAP IDLE altogether. Typical.