Gambling Law
If a casino accepts bets from a US citizen, they might be either breaking US law or perhaps their own licensing agreements. Many casinos don't accept bets from US citizens or from other jurisdictions that are attempting to prohibit online gaming just to remain on the safe side and to avoid any possible prosecution. It's in the casinos' own best interest not to accept these bets. However many online Casinos DO accept bets from US and Canada.
To date, there are no federal laws that will prosecute online bettors. There are laws that prohibit online casinos to accept bets (that's why they are liscened in countries where it is legal). But there are some states that do have laws passed: California, Florida, Indiana, Illinois, Kansas, Louisiana, Michigan, Missouri, Nevada, South Dakota, Tennessee, Texas, and Wisconsin all have some form of a law that prohibits online gambling. The best bet - no pun intended - is to check with your local legislators to find out if it's okay. Or better yet, call up your local police department to see if they even care.
Again this pertains only to those who live in the states. Obviously each country has its own laws; in the UK its perfectly legal, in fact encouraged, in Germany it's much like the states; please check with your local jurisdictions to find out what's up
http://www.casinoportalen.com/asp/Gambling_Faq/Gambling_law.asp