Chopstix Piano Bar
11 Roy St
http://www.chopstixpianobar.com/
This place is pretty cool. The performers/piano players were pretty good and fun. I'm not sure this is a 'Dueling' piano bar for lack of piano dueling. The place was a little small and seating/standing room is limited. You can call ahead for reservations but you must get dinner there. Friday and Saturday night are busy so I would recommend getting there early if you want a table.
Another opinion:
We went once and it was a lot of fun, but I spent $175 on drinks. Pre-funk in the parking lot! The pianists were great, and they hold what could be considered an "auction", you bring up a request with money and they'll play it, anybody can stop your song to play another if they "outbid". I recall that somebody wanted the theme song to "Mash" played, and payed over $60 to hear it through. This type of action makes for quite an entertaining evening.