So, in order to get something resembling hard numbers, Dredge took the reported sales of a few popular App Store apps, and then he compared those to the number of players listed as playing them in Game Center. You can see the top few examples above -- keep in mind that this is a picture painted with a roller rather than a fine brush; it was just created to give an idea of what's happening rather than exact figures.
But there are some conclusions to draw -- a title like Angry Birds, with over 6 million players, is only seeing about a quarter of those users using the Game Center integration. At the same time, a newer title like Cut the Rope has a majority of its players using Game Center. In other words, Game Center is still in the early adopter stage -- the mass market of iPhone users hasn't yet jumped on board. But users who seek out and find the latest and greatest apps (and Cut the Rope is a hit that's only a few weeks old) are signed up and playing on Game Center already.
That's pretty fascinating -- if I was a developer, that would tell me that it's probably not worth implementing Game Center on my older titles, since players who only stick with tried-and-true apps probably aren't using it anyway. But at the same time, I would say that any new apps should most definitely use Game Center -- the App Store's quickest users are looking for its implementation in the apps they choose to play.
That's not to say a game can't compete and succeed without Game Center -- a good game is a good game. But it does mean that simply putting that Game Center icon on iPhones hasn't yet given Apple a widespread hit. There's still a large number of (likely very casual, "non-gamer") players out there who haven't signed up to play around with Game Center's multiplayer and achievements features. And that's like a prime reason why we haven't seen any quicker adoption from iOS devs