iphonePOV recently discovered that the hole for the headphone plug on the top of the iPhone is recessed into the device, and as a result, nearly all 1/8" (3.5mm) headphones and accessories cannot be utilized without an adapter. While many users are content with the headphones that ship with the iPhone, many Audiophiles prefer higher-quality, over-the-ear headphones, and will be annoyed and surprised when they attempt to connect their overpriced headphones to their overpriced phone and realize they'll have to spend even more money to make the two parts fit. Belkin is the first out of the gate with an adapter which is conveniently overpriced as well.
Side note 1 -- if you're looking to make a buck on the iPhone craze, forget selling overstocked phones on ebay, and instead, figure out how to get a headphone adapter on the market for a reasonable price.
Side note 2 -- This is not the first time Apple has modified an existing connector to control which accessories can easily connect to their machines. e.g. The USB connector on the end of the white wired keyboard that ships with the G5 and iMac has a small notch in it which allows it to only fit into Apple USB ports.