[Cerril]

That's a complicated question. If you were to deal out 100000 hands at random you'll see as many 7s as As on the board, but if you were to look at a 100k sample of actual hands you'll probably see fractionally more aces for the sheer fact that people chase aces more and play hands with aces harder on average, so more hands that are played to showdown will involve a paired ace. It's not going to be a huge disparity, but I bet it'd be there.

That said, about 35% of the time you'll see an ace by the river in boards with totally random hands, 35% of the time you'll see a seven, and about ten percent (plus or minus)you'll see both an ace and a seven. (so figure it's split -very- roughly along a 25/25/10/40 line for A, 7, A+7, neither).

Or in a more telling way to look at it, a reasonable amount of the time someone pairs up their 7, someone else pairs up their ace.