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.
