It seems the first wave of enforcement has come from HOAs, I've heard of various people having to remove their listing for that reason. [1]
Now it appears, according to the article above, municipal authorities are pushing back. Could this be a result of lobbying from hotels? Vacation rentals is not a new concept, and couchsurfing has been around for years as well. It seems that just because couchsurfing/vacation rentals are getting a wider audience that it's now suddenly a problem.
I personally don't want my HOA or government telling me who I can or can not have over to stay. Their place is not in my home.
1: http://betabeat.com/2011/05/airbnb-takes-manhattan-with-2k-b...