Before the iPhone 3G came out in July, I did a quick price survey on the 1st generation iPhones being sold on eBay.
A quick search reveals that used & unlocked 8Gb iPhones are going for ~$400 and 16Gb for upwards of $500, with never-opened phones going for even more.
After the 3G came out, the prices on the old iPhone remained about the same.
I just checked eBay again and those prices are down only slightly. Never-opened unlocked iPhones are still fetching $400-500 and somewhat less for previously used phones.
BusinessWeek recently confirmed that those old phones are still selling well, demonstrating a lot of demand for iPhones that can be easily unlocked for use on networks besides AT&T in the US and elsewhere in the world.
On e-commerce site eBay, where NextWorth peddles many of its wares, a 16-GB version of the first-generation iPhone goes for about $600, and an 8-GB model in good condition commands $500. When it was new, the 16-GB phone sold for $499; the 8-GB model went for $399. Today, AT&T's most expensive iPhone 3G model sells for $300 with a two-year service contract.