I wouldn't mess around colluding live. I've never seen it happen, but I know I would be real pissed if I found out some jerkoffs were cheating. That's how you get beat up or even worse. There are a lot of crazy people out there. Money makes people insane. Personally, I value my health more then a couple of bucks.
Ok, stepping off the soap box, I think it would be difficult to actually collude in live games and be successful. You'd have to be really good at signalling. This isn't a movie like Rounders. This is real life and people aren't that dumb.
Colluding online is a different story. You can easily open up an instant messaging client and tell a friend or two what you have and what you are doing. This is what scares me about online poker. Sportsbook.com has pretty good security when it comes to this. I've colluding a few times in cash games (I know, I'm scum, but I was curious), and I've transferred money from one account to another by playing a friend online (sportsbook does not let you transfer funds). I've been caught twice by Sportsbook (the 2nd time I was real lucky cuz I had about 2500 in my account) and both times I convinced them not to disable my account.
Colluding is tempting, but you really need 3-4 people to do it correctly (2 people playing as 1 doesn't affect a 10 man game as much as you think). The more people, the less you collect. It really isn't worth getting your ass beat or losing your roll IMHO.