http://www.wsgc.wa.gov/FAQ/InternetGamblingFAQ.htm
Here is a very plain English webpage talking about the legality of online gaming. The rules vary from state to state but my last job was for a large online casino and even though we were based in the US, we did not take bets from the US. As you'll see in the above FAQ, it is illegal to solicit bets from inside the US because it violates the Federal Wire Wager Act which was designed to stop racketeering.
Cyn