As a TA for a few years I've learned something: if you cheat, you'll likely get away with it. First off it is really hard to catch people cheating, especially when you have 300 people in a room. If someone is caught what usually happens (at least in my department) is that the professor will give an F for that exam but do nothing more; the funny thing is that they usually drop a midterm anyway so it has little or no effect. And most professors feel like it is impossible to gather enough evidence to pursue academic action. In my department, at least, the head lecturer is pretty soft when it comes to cheating; if a professor catches a cheater and confers with the head lecturer, he'll talk the professor down into doing nothing. I've heard, though, that our department is unusually soft on cheating so you probably shouldn't extrapolate too much.
The most common way of cheating is to copy answers off your neighbor; but when you're grading 300 exams it tends to be hard to see if two people have the exact same answers--especially since the each TA will grade only 1-2 problems for each of the exams. The only case where we can detect cheating is when the answer is so egregiously wrong that we joke about it then see the same egregiously wrong answer elsewhere.
Now if a student is actually caught and disciplined it isn't that bad; usually they'll take a seminar course and that will be that. I've even heard that the local med school where I'm at will ignore disciplinary actions for cheating if it happened in the first year or two.