At the least, the Penny Arcade comic, or anyone else who posted about Wi-Fi on a MP3 player can be prior art in a 35 USC 103 (obviousness) rejection. In my short time as a patent examiner, I rejected plenty of claims as obvious. The rejection requires disclosure of two or more items that, when combined, would amount to the claimed invention, plus some incentive to combine the parts as per the claims. The comic would, at least, provide the incentive to combine.
I do not think it matters, though. Doesn't ThinkGeek already sell a MP3 player that allows purchasing music over a WiFi connection (MusicGremlin)?