Techcrunch posted a story earlier today about this, and will probably update more than the one time so far. This is developing... @jonlech just posted a link from a tipster of someone (a cop?) tearing the ad down. He also mentioned that the ad would be back up on Tuesday, lamenting that it would not be up on Monday for the anticipated iPhone announcement. WWDC does go on all week though, and I bet that doubleTwist is going to get a prorated discount since the ad will have been down a couple days.
~Chris