The server is reportedly having some start-up issues, including connectivity and crashing issues, but developers who were limited before are now able to create full accounts and full-on matchups. Currently, Game Center is live in 4.1, which we expect to arrive soon. Developers can submit Game Center applications to iTunes Connect using the 4.1 GM seed.
Once the app and iOS 4.1 are available, we'll have a full review.
Thanks to everyone who sent this in!