Infinii, I had one of my best friends just go through a federal court case in the last 3 months that revolved around a gambliing ring. He had to testify on behalf of the Federal Gov., so I learned alot about what you can and can't do.
As Loophole stated, the only legal sports gambling you can do is in Nevada. All of the office pool 100 square Superbowl games and NCAA tourney bracket contests are technically illegal. Don't think they have the manpower to police all of those though. It is illegal to take a bet and charge juice, as the locals do, but if you make a bet with someone and their is no juice involved, I have been told that this is fine. I know of a group of well-to-do business men that get together for lunch every Friday during the football season, and make bets among themselves, with no juice involved, they just cover each others bets, and they don't get hassled at all. If you have some friends that you are placing bets for with a local, as soon as you take those bets you are breakinig the law according to the FBI. In their eyes, you then become the broker, whether you hold them or pass it on, doesn't matter.
Its getting late and I can't remember everything I learned in the last 3 months to pass on. Good Luck.