This is the message I got from BetFair when I tried opening an account:-
An error has occured while processing your request.
Owing to legal restrictions, Betfair does not accept bets from within the USA. We have stored your details and will contact you in the case of a change in policy. If you require more information, please contact our Helpdesk on 0044 20 8834 8060 or email info@betfair.com.
Haha, I made the same discovery with Eurobet. I got an email that they put a free $10 in an account that I had somehow created but never used. I went to place a bet but some bizarre error message kept occuring, so I emailed CS, and they informed me that they don't take bets from the U.S. Too bad the cryptic error message couldn't have told me that in the first place. I ended up taking the 10 bucks to the poker room, got it up to 30 bucks before getting jacked by a ridiculous run of suckouts and lost it all. Easy come, easy go. At least it was free money in the first place.