Apple hasn't thus far allowed 3rd party mail applications, including Gmail clients, into the App Store.
I believe iPhone 3.0 SDK will allow for the embedding of Mail (like it allows Safari to be embedded already), so maybe that will change things?
However, most of us want Push Gmail, which would require more than just a Gmail app (even with Push Notification Service), but likely some hard work on both Google and Apple's end to integrate it into the built-in options.
I know many would like a separate app, like the Android G1. I'm not a fan of that idea. I want all my mail in one place. So Push Gmail added to the existing Mail app, with all accounts getting stars/flags would be just fine for me.