I'm almost positive it was Chris Rock. The "hooker" thing was the kicker and I don't think he elaborated on it any further during that particular show. That being said, pretty much every comedian has made a Viagra observation, so there's a lot of material out there.
Which makes me think: we should do a "Who's the best stand-up comedian?" thread if there's not one buried somewhere. There are so many good routines from Pryor, Murphy, Carlin, etc.