It seems "Crackdown" isn't just the name of a popular Xbox 360 game but it's what MS is doing to people who have tampered with their gamesaves to earn a higher gamerscores and get more achievements.
It goes something like this...you see a post on a message board from somebody who is offering to play "Halo 3" for you so that your gamerscore goes up. All you have to do is send him your account info, so he can sign in as you, and paypal him $40.
So, what if you live in a dorm and there is one Xbox 360 where four of you sign in. Don't worry. It's not so much lots of accounts on the same 360 as it is your account on several 360s. Add into that the wizbang technology that MS uses to determine if you're earning stuff a little too quickly and/or tampering with saved game files to undermine the system and you're toast!
On the one hand, I'm of the philosophy, "so what?". It's none of MS's business what I do with my account. On the other hand, fair play makes games more fun.
For more on this, visit Major Nelson