Mobile Crunch has posted a few pictures of Apple's Game Center app
(still only available to developers and therefore still under NDA), and it appears that Apple is still working on the look and feel of the app. Previously, they were going with the dark blue look you can see above on the left (though I'm not convinced that wasn't just a placeholder), but the latest update
has a much more colorful and textured look to it, almost like a felt game table or board game set.
I suppose it's also interesting that you actually need to sign into iTunes from the app -- I don't know if Apple would be able to simply sync everything up all the time, but I'm still entering my password in the App Store app every time I buy something new, so I suppose that would follow.
Game Center is still due out sometime this fall, and Apple is still testing and developing it behind the developer NDA wall right now.