I've been having the same problem for the past week. I'm on an iphone 4 and I've tried deleting the app and reinstalling it. Nothing happened. I tried with Gamecenter disabled and enabled. Again, nothing. Also, I'm in Canada and I hope that it wouldn't be a factor.
I'd rather not have to restore the factory settings, as someone else on the board has suggested, so I'm hoping you guys might have better suggestions or any news of an update or patch coming soon.
Love the game though and can't wait to play more. Please give us some good news!!