It's real...found this: ''When tourists come to the Big Apple, sometimes they are oblivious to our cultural habits. It may be socially acceptable to smile at strangers in places like Florida or California or Chicago or the rest of the country, but not here. Please allow us to read our copy of The Girl Who Kicked the Hornet’s Nest minus the creepout. If you plan on visiting New York, please be sensitive to our needs.'' http://subwayartblog.com/2011/02/21/tourist-information/..so,as far as I can tell..
It's satirical "street art"...a clever counterfeit made to look like a genuine NYC subway sign...the humor is more subtle than most...