We first QoD'd on this popular form of entertainment in August 2005, although we've updated that original answer a couple of times since. Still, this seems like a timely time to revisit, not least because the Orleans recently announced that Brendan's Irish Pub will be hosting a limited run of dueling piano shows at 9 p.m. on Fridays and Saturdays, commencing July 4 and ending August 30.
As to the original answer, the line-up hasn't changed that much in the intervening years, save for the discontinuance of the dueling pianos at Porch Dogs at Hooters, (which also had dueling guitars), the reinstatement of shows at TI's Kahunaville (which had ended in Oct. 2005), and the addition of a show at McFadden's, the new Irish pub at the Rio. Here's the current lowdown on venues and schedules:
Harrah's Venue: Harrah's Piano Bar The Twins Dueling Pianos Show Times: Sun-Thurs, 9 p.m.-2 a.m.; Fri & Sat, 9 p.m.-3 a.m. Cover: None Tues-Sat features twins Kimberley and Tamara Pinegar Sun-Mon features a set of male twins whose identity remains a mystery. (Let us know if you know...)
New York-New York Venue: Times Square Bar Times: Thurs-Sun, 3 p.m.-7 p.m. / 8:15 p.m.-2:15 a.m.; Mon-Wed, 8:15 p.m.-2:15 a.m. Cover: $10 after 7 p.m. (daily)
Paris Venue: Napoleon's Times: Mon-Sun, 9 p.m.-1 a.m. Cover: None
Rio Venue: McFadden's Times: Sun-Tues, 7 p.m.-10 p.m.; Wed, 8 p.m.-11 p.m.; Thurs-Sat, 7 p.m.-10 p.m. Cover: None
South Point The Rockits Venue: Del Mar Lounge Times: Thurs-Sat, 10 p.m.-2 a.m. Cover: None
TI Venue: Kahunaville Times: Mon-Wed, 10 p.m.-1 a.m. Cover: None (and also features the exploits of their famous flair bartenders).