Ummm, why? Obviously you'd only bet if you were planning on claiming the attack publicly shortly afterwards, so maybe you'd see some bets from Bin Laden and associates. If say a CIA operative had prior knowledge of the attack, do you really think they'd risk placing a bet? Besides, what would they even bet on? "Will there be a surprise terror attack that nobody expects in 2001?" or "Will the US suddenly decide to invade Afghanistan" or what?