Michael,
It's actually somewhat in dispute. The Justice Department says all online gambling is illegal if it's across state lines. (States are allowed to regulate themselves -- you could potentially have a state say online gambling is fine, and as long as only those state's residents are gambling online casinos in their state then it's no problem). I'm not sure about serving people overseas from a U.S. concern, but I'd think that would be very difficult to enforce from a customer standpoint -- what would you do, check IP's?
There's also a question of technology. The Justice Dept. is currently in an interpretive battle with the horse racing industry, for example, about whether or not the Wire Act (which prohibits interstate betting over *phone lines*) applies to the Internet. Clearly the spirit of the statute does, but it also expressly says telephone lines. And as we all know, people don't use phone lines much to access the Internet anymore.
ripley.