I'm running Google Apps right now on my Blackberry, so I get push e-mail through Blackberry's servers, and the calendar and contacts are synced by Google's sync app which runs in the background on the BB.

Google offers the same setup for S60 Nokia devices, so maybe we'll see a port for the N900? That would be fantastic - use Nokia Messaging to push your email, and then use the Google Sync app to update calendar and contacts.