The game is simulated on every PC. Data is sent P2P. The host "syncs" the game by making sure everyone has completed the previous "round" before they simulate the next one (with the rounds happening so fast that you think it is real time).
As such, if one game is hacked, the others are not, so you end up with a game that is out of sync, which would be captured by the checksum data, resulting in a mini-dump (game crashes and creates a log file called a MD)....
So I wouldn't worry about cheating. The only thing you have to worry about with MP is people that are so good you think the only way they are doing what they are doing is by cheating (but they aren't).
BTW - Any exploits that get found are quickly patched out of the game.