It won't be fixed. Because modest, the program used for email on the N900, has IMAP IDLE disabled, for good reasons I suppose (battery impact?). And Nokia won't tinker with modest, the modest-guys have to tinker with modest. And they won't tinker with it. So no tinkering, means no fixing. And without IMAP IDLE, there is no "generic push mail" only the solution offered with Nokia Messaging (which I suppose is mobile-usage-optimised). But speaking of that: why don't you use Nokia Messaging? Work's ok with my Gmail account here.

Speaking of the iPhone: I'm not quite sure if the iPhone really uses IMAP IDLE for Gmail, I'm rather suspecting the iPhone to do exactly the same what Nokia Messaging does on the N900, it just doesn't let the user know it's using Apple's servers instead of a direct connection to Google. But that's only an assumption on my side which, looking at Apple's business practices and the-user-is-dumb-approach, looks fairly reasonable.