In the keynote, i believe they specified that it has to be a Series 3 to work on the power mat
Edit: It's at 1:47 of the keynote. he doesn't say that it must be a Series 3 but he does say "You can place your Series 3 watch on it..."
I wouldn't be surprised if this is exclusive to series 3, but it's not clear to me from how he stated it in the keynote.