Much appreciated! I was taking N to be the number of squares on the board. My current thought is that, as you said, the number of possible move sequences on an N square board is of the order of N^N (actually, I think slightly smaller: N!). As you said, N may be much larger than the number of stones already played.

My current understanding is that board size if fixed for any given Go problem. Is that true or false? If it is true, then I'd think that the factor of N branching at each step in the tree of moves is just what gets swept into the nondeterministic part of NP.