In their Saturday update for The Sacrifice, Valve has announced that 5 new mutations are coming, including TAAAANNK!, a versus mode where every player on the infected spawns as a Tank, and only a Tank. I personally can't wait.
There are five players in the lobby. One person is the Tank, the others are the Survivors. The person controlling the Tank will respawn as another Tank upon death (and keeps going that way until the end of the round). When all four Survivors are killed, the round ends, and the Tank control rotates to another player. The winner of the game is determined by how quickly the Survivors are killed for each round.
If it's 4 normal Tanks then the survivors will win almost every round. Seriously once you juke past 4 tanks no one is going to catch the survivors. I am with Mike on this I hope Valve thought ahead otherwise there is going to be a ton of rage quitting.
A better idea would be to have it that after 10% into the map a Tank spawns and after that Tank dies a new Tank spawns. This repeats until the end of the map. That would be legitimately hard for the survivors.