(just checked the app store and AT&T GPS app is still around and they charge you 9.99 a month).

My thought on the issue is if you don't want to rebuild the game completely do that players but an ingame subscription (such as done in Chaos and Order), and apple does not allow you to use subscription thru the app store; simply set up a use login on your webpage for paid members.
I think that's been a way all p2p MMO's been done before app world...
You register on a webpage, pay on the webpage, and play the game via game client (which in this case would be an app).
Let me know what ya think about my crazy ideas