i blogged a bit about this but thought i would post as well.

I'll start off with posting a link to another grinders blog. http://daleroxxu.blogspot.com/

he made a huge post about 49 colluders cheating in sngs, mostly DON's. very long read but good. it caught these players colluding and took other players for over $500,000. there is a list of players there that colluded, in a ton of games that i was a part of and definitely cost me some money.
just got an e-mail back from stars when asking them about this case and what they do to detect cheaters, their response:

Collusion is something we do not tolerate at PokerStars. We have several
security measures in order to detect collusion. These include, but are not
limited to, the following:

1. Tracking IP addresses and matching historical playing patterns. This will
flag and determine if certain players historically play on the same table.

2. Strong hand folding. If a player folds a strong hand before the flop
such as KK when another player has AA it is flagged.

3. Any player reports of a suspicious situation is thoroughly reviewed by
our poker experts. They will also review the historical play of the players
involved. Because we have access to see every player's
history, it gives us the upper hand in detecting collusion better than any
live casino.

Obviously 1 and 2 can do wonders but what is the best way and always has been is #3, it is our job as players to police all of our games, if you ever notice anything suspicious you need to send an e-mail to the poker site letting them know. this will make all of our games in the future much safer.