For Gmail users, the email experience on the iPhone has never felt as good as it does on Android. Although you can hook it up to the iPhone's email client using Google's licensed Exchange functionality, native Gmail functions like starring, archiving, and search haven't been fully integrated. That could change soon, as MG Siegler is reporting that "multiple sources" have told him that Google has developed a Gmail app for the iPhone and may have even submitted it for review. The main advantages over Gmail's HTML5 implementation would be true push notification for a native Gmail app and what would likely be faster and easier access to those Gmail functions.
If this rumor pans out, it would be a big deal for Gmail users, but in the meantime we'll just have to wait and see (and yes, the above image is just a mockup).