BoyGeniusReport and Engadget (sourcing Pre Central) are both reporting that:

the Pre will, in fact have IMAP IDLE (AKA Push) Gmail capabilities,

Let’s get one thing straight: IDLE is not Push

IMAP has been around for awhile, and so has the IDLE extension. This allows you to login to your IMAP account and remained logged in while idling, and notifications of new mail are sent to the client. Note that this does not push the mail out itself, just the header, with message marked as new so the client can then download the mail when initiated by the user. I use IMAP with IDLE support on my mail server, and it is fantastic, yes, but it’s not “Push” mail. What Apple and the entire rest of the world mean when they say Push mail is that the entire message is delivered to the client instantly after the server receives it. Push requires more bandwidth, a bit more CPU usage, but most importantly a protocol that is not IMAP.

In my research there are only a small number of ways to achieve Push mail. Microsoft Exchange does it, and while I don’t know what Yahoo Mail is doing or what Apple is using for MobileMe Push, it’s more akin to what Exchange is doing at the protocol level and definitely not IMAP IDLE. There is also some Java based Push mail server implementation available from Sun, but as far as I know it is not open source, or even free of charge. To my best knowledge there is currently no open standard for pushing mail. Believe me, I’d love it if there was and I would be the first person installing “GNU FreePush” or somesuch on my personal Linux mail server if anything existed. But for now, IMAP IDLE is poor man’s Push until there is such a standard. In my crazy imagination, Apple is currently using a homebrew Push protocol and server and they’ll release the source along with Snow Leopard Server.