Target has issued a series of ads in the New York subway system for a live album by singer John Legend.  The ads feature a headphone jack where you can plug-in and listen while you wait for the next train.  Check it out:

Taking advantage of the user’s situation (bored, waiting, and likely having headphones in his pocket) is a beautiful example of contextual design.  My only concern is that if I have headphones, then I likely have an mp3 player as well.  If I’m already listening to music I like – why would I switch?  The design banks on human curiousity, counting on people to opt-in to the novelty of the idea.  I’d love to run some tests and see how well it works.