The ad invites passersby to try "The Cure for iPhone Envy", which they can use to access their "iTunes Library on any device. In Seconds". It's clearly a message that Apple doesn't want anything to do with. We're hearing that Apple employees are currently scratching their heads as to how this appeared.
Apparently the window technically belongs to BART, the Bay Area's commuter transit system. doubleTwist got in touch with an ad agency that BART deals with and leased the window, giving them the chance to plaster their ad just below the Apple logo in its full glory. This is apparently the first time the window has been used for this purpose (before it just sat bare). And because everything was done legally, Apple's going to have a hard time getting rid of it.
"Don't tell me I'm burning the candle at both ends -- tell me where to get more wax!!"