Really good cheaters, working as a team, are the most serious threat to either casino or online Poker. Given that cheaters don't usually make the best partners, it's rare in either type of game, online or offline, for a pair to make a good job of it. Small-time cheaters who exchange info over IM or the phone (but don't have a common bankroll) are still cheaters, but not a real threat. You'll be able to "flag" this type of cheating, even if you have no proof, and the machines will do the rest of the figuring.