From onlinepokercenter.com:

Should I be worried about player collusion?

Collusion occurs in both online and real life games. Fortunately it is much easier to detect online collusion. Online poker rooms analyze each hand history with sophisticated algorithms to detect suspicious betting patterns and player interactions. Moreover, online poker rooms also monitor IP address to ensure people from the same location do not play at the same table.

Many of the classic poker cheats used in live games do not work well online. A good example is “chip dumping”—the practice where one player intentionally loses his chips to his partner during a tournament. In a live game, chip dumping is impossible to detect once the loser mucks his hand. By the time you complain to the pit boss, the dealer has already shuffled the cards, erasing all evidence of the chip dump.

It is a different story online. Every single hand, even the mucked hands, are recorded and analyzed by security algorithms. If two players have a pattern of making ridiculously bad bets against each other during tournament play, the computer will detect it.