That's incorrect. IMAP does in fact support PUSH, in conjunction with IDLE, and Dreamhost supports it for the explicit purpose of mobile devices like the iPhone. Read here:
Also, in this thread d00d mentions getting Dreamhost PUSH to work with an iphone:
"Actually, based upon some informal testing with Dan Moren's iPhone and an IMAP account with IDLE support (Dreamhost), the "push" email support is simply the support of the IDLE command."
So, any ideas on getting mail pushed out to my iPhone?