Before the iPhone 3G came out last month, I wrote about how valuable the old iPhone still was.
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.
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. If you've purchased an iPhone 3G in the past few days, you still have an excellent shot at getting most of your money back from your first phone (provided you can get it unlocked, which isn't difficult).
I also checked the prices for unlocked iPhone 3Gs...prices are upwards of $1400 for the 16GB model. The unlocked claim is somewhat dubious. AFAIK, there hasn't been a crack released yet although it's been reported that the 3Gs are being sold unlocked in Italy and Hong Kong.
Update: The 3G has been cracked.