This Business Week article describes the rising demand for first generation iPhones. "Shelf prices" (using eBay values of virtual shelves) for the 16GB iPhone are currently at about $600, and $500 for 8GB models. In other words, the $250 AT&T refurb specials from March are now worth roughly double what you paid for them.
From easy-unlocking to contract-free usage to eco-consumerism, consumers are looking for those original units and paying premium prices to get them. A majority of phones are heading outside the US for resale while growing demand at home keeps resale prices high.