Dominique Strauss-Kahn was officially arrested this morning so French police could question him about his alleged involvement in a prostitution ring. His lawyer helpfully explains that DSK did not know the ladies he was reportedly banging at sex parties in 2010 and 2011 were professionals, since they were not wearing their Official Prostitute Uniforms: "He could easily not have known, because as you can imagine, at these kinds of parties you're not always dressed, and I challenge you to distinguish a naked prostitute from any other naked woman." It's good to know that DSK's legal team is as classy as he is.
I don't really know about French laws in this area (in fact, I thought prostitution was legal in France) but I'm pretty sure in the US you have to prove that they did in fact intend to do the act, i.e., pay for illegal services. And I imagine DSK was not the one paying these woman (at least not directly). So the prosecution may actually bear the burden of showing that he intended to have sex with a prostitute... Not that this is a great defense, but proving mental state without other evidence (evidence of payment, for example) can be a pain.