GPush is a newly approved iPhone application that, very simply, adds Gmail push notifications to your iPhone. (If you're unfamiliar with the concept of push notifications, GPush notifies you the instant you receive a new email rather than after your Mail application checks for new email [or pulls] .) To use it, just fire up the app and give it your Gmail username and password (your login credentials are encrypted with SSL and sent securely to Google). After that, you'll get push notification (complete with the From and Subject fields) any time you get a new message in Gmail. GPush allows you to toggle sounds, alerts, and badge notifications, so you can determine what level of alert you want.
The catch: GPush will set you back $US1 at the App Store (for now, at least—it goes up to $US2 eventually). Still, a one-time price of $US1 isn't bad for the potential time- and battery-saving advantages of push Gmail on your phone.
GPush [iTunes App Store]