I'm sure the joke responses are deliberate, just as Infocom text adventure games were programmed to be 'smart' enough to respond to predictable human input.
I posted to Quora yesterday to find out who exactly (besides Wolfram Alpha) is behind Siri's impressive sense of humor.
I can't help but think it's more likely that these images are faked than that Apple actually designed Siri to recommend escort services.
It's not like they had a bunch of people sitting around a conference table thinking up the phrases people might ask it. It's a natural language parsing engine; it understands "horny" as sex, and pulled up related Yelp listings.