Python 80 (69) char
Not the shortest Python solution, but I like how it introduces "DICE" into a game of tic-tac-toe:
W=lambda b:max([b[c/5-9]&b[c/5+c%5-9]&b[c/5-c%5-9]for c in map(ord,"DICE>3BQ")])
69 chars for the simpler expression:
max([b[c/5-9]&b[c/5+c%5-9]&b[c/5-c%5-9]for c in map(ord,"DICE>3BQ")])
