As long as Gavin holds the key to the 'panic button' for the entire network (which I don't think is necessarily the worst choice), he will have slightly more power over the direction of bitcoin than anyone else.
It's a key built in to the network that essentially gives him the power to broadcast a special alert signal to the network. I'm not actually sure if most clients are equipped to interpret these signals now, though.
edit: I should clarify it's built into the original satoshi client, not necessarily 'the network' and certainly not the blockchain. Someone on bitcointalk explained it as "part of the peer to peer overlay and not part of the blockchain. You can find this public key in the source code of the official client if you look."