You can have a cell phone in your pocket but they won't let you use it for anything. I suspect in the era of smartphones that they would completely ban them from the floor if not for the fact that enforcing that rule would be a nightmare.
Source: I've absent mindedly taken a call while sitting down at a table in Las Vegas and got an instant warning from the dealer.
Source: I've absent mindedly taken a call while sitting down at a table in Las Vegas and got an instant warning from the dealer.