Yeah thats true. I tried to use gamecenter when i was jailbroken on 4.0 but it didnt work right so i just said eh, Ill give stock 4.1 a try before the jailbreak comes out in like a week.
I had the same problem until around 10 seconds ago
I just bought Flight Control and it wouldn't show up on the GameCenter
but give it like a minute or two and it'll eventually be there