According to the Las Vegas Advisor Online Issue posted today there are four pools in Las Vegas allowing topless or European style bathing:

Caesars Palace, must be a hotel guest

Mandalay Bay, must be a hotel giest, women $10, men $40, but $50 on weekends

Stratosphere, hotel guest, men $25

Wynn Las Vegas, hotel guest
Surprisingly, no Hard Rock and no Palms, despite the "Skinny-Dipping Tuedays" billboards around town.
Amazing that MBay charges women, with the prices they charge men. What if there were no women, would the men still pay those steep prices?