Wow. There's so much wrong information in this thread, I don't even know where to start.
The comparison to Verichat, the mentions of SMS, none of that has absolutely anything to do with the Pre's email client. There are a few other clients that use this method, true, but not the Pre.
The ONLY way that the Pre gets pushed new messages is through the IMAP IDLE command. If it's getting pushed new messages any other way, that's not the IMAP IDLE command.
"Who knows how IMAP IDLE is being handled?" Uh, there aren't multiple ways to handle it, it's a specific protocol.
And push DOES use negligible battery for some, for exactly the reason oddlou states. If your email is set to be pushed, it connects once and then stays connected, yes, but it's not actively transferring data. If you set your email to check every ten minutes, it's actively transferring data every ten minutes, which uses data more than staying connected via IDLE. But if you're connected via IMAP IDLE and get emails more than every ten minutes, then yes, that'll use more power than pulling them at a ten minute interval.
Either one will use more battery than not checking at all, of course, but if you don't get many emails, push can be more efficient.