Although I have seen tanks last for years situated like that I personally wouldn't risk it, imo you should always have the sides of the tank supported and a support beam for every foot of tank.
OK guys, thanks for the replies. Guess I'll stick with a tank that will sit on the sides...I'd chance it but it's in the living room and if there's a disaster the wife will kill me. Maybe I can find one of those 33g long tanks and put that down there...