The answer is... maybe.
It's not legal for gambling websites to be operated in the US. But it is legal for them to be operated in other countries. Those sites will tell you that it is legal for you to place your bets with them as the money and operations are offshore, therefore not subject to US jurisdiction. The problem is if you get cheated, you can't count on the US to step in and help out.
The other issue to consider is how to get the money to those offshore sites. Most credit cards and banks specifically prohibit gambling related transactions.
Another thing to consider is what happens if the US decides that the site you are using is illegal and shuts that site down and freezes the assets like they did with some poker sites recently. Then your out of that money until such time as they decide to give it back.
Long story short, the biggest gamble on offshore gambling sites is actually using them.
“…people’s need to be right is stronger than their ability to be objective.”
N. Crawford, The American Psychological Association