All miners connected to the pool are mining for the pool. So if anyone finds a block the block will belong to the pool (gets paid out to the pools address).
Just a quick correction here... not all pools mine to a central address and then distribute payouts - although most do (including my own pool). P2Pool and Eligius split the 25
BTC up in the coinbase transaction itself so when the block is found, the miner gets virgin coins.
P2pool always, eligius ... most of the time.