What's with Foursquare's Times Square ad?

It's been a big few days for location-based service Foursquare, which just hit 3 million users and also became the centerpiece of this gigantic advertising ziggurat in Times Square. (Larger image after the jump.) It's unclear what kind of deal made this ad happen, since it seems unlikely Foursquare could afford such a high-profile placement. On his blog, Foursquare's Tristan Walker thanks the sign's owner, apparel company American Eagle, but also says the ad was designed by Foursquare's own designer, Mari Sheibley (aka, the woman who draws the badges).