Some gamers still prefer PS/2 if given the choice, even though there's little technical advantage. Although their most cited reason (key rollover) isn't valid; the keyboard determines how many simultaneous key presses it will transmit over USB.
PS/2 keyboards observationally are known to have a bit less latency. Since there are a ton of variables at play (keyboard controller, USB poll rate, PS/2 interrupt queuing, etc.) it's hard to evaluate just how much the difference will be with any given setup.
http://geekhack.org/index.php?topic=5572.msg85020#msg85020
I'm sure a keyboard could set up their own driver that allows more key presses, but that's certainly not the standard.
http://www.maxkeyboard.com/max-keyboard-nighthawk-x8-blue-ba...