Sad news for iPhone owners. Gamecenter is coming later this year. It's a bit later than most of us had expected. It just seemed like a major part of iOS4. Heck, it was one of the "Tentpole" features, and it was either cut from the current build, or Apple truly meant to hold it over and just forgot to mention the specifics on that. After all, they did introduce it to the public during the very same event in which they introduced all the other features of what was then called iPhone OS4.
Looks like we will have to wait a few more months. Maybe it will come when the iPad support drops.