Didn't it take Apple a year or two to add IMAP idle to the iPhone?

Anyway, just use Nokia Messaging to connect gmail to your phone, as mentioned above. It works very quickly, and has other advantages. e.g. you can set it to not disturb you at night. (regular IMAP on n900 lacks that too)

My problem is the lack of proper contacts/calendar sync to gmail. And that is google's fault really, for using Exchange.