iOS 4.2 isn't quite out yet, but 9to5Mac has already spotted some Game Center-enabled iPad games out on the App Store. Big Bad Sudoku Book is an iPad app that's apparently good to go already, presenting the Game Center icon with its listing.
Of course, without the actual firmware, there's not much to do with any leaderboards you top or achievements you earn -- for those, you'll have to wait until the actual iPad firmware to arrive (which it should soon, at some point this month). But given that Game Center is integrated with an iTunes account, achievements earned on your iPad should show up on your iPhone, and eventually vice versa.
It will be interesting to see, once the service is all rolled out across both platforms, if multiplayer works between the two devices as well, or if there are any other little quirks involved in using Game Center in both places. As long as there are Game Center-enabled titles showing up right now, there probably isn't much longer to wait.