Just a preliminary thought based on a few observations on my Mac…
Using IMAP with Mail.app on Mac has CPU at 0 - 0.3% on idle monitoring an Exchange and a GMail mailbox (settings: Check every 15 mins, Use IDLE if server supports it).
Using Mailplane to monitor GMail (web app inside nice Mac interface), CPU 3.0%. One can close the windows without closing the application and get this down to 0.2-0.5%.
Using Safari to monigor GMail CPU 5-7%. (Firefox show similar if not slightly higher numbers).
Seems like there are battery discharge time considerations here for mobile users (like me).
Change note: Re-reading today (21 June 2009), changed title from “Web …” to “Mail …” to match the content better.