Erica Ogg on CNET has a round-up of places tto sell your pre-iPhone 4 iPhone (i.e., last year's--or the year before that's model). The prices vary, so it's worth following the links.
And, as she points out, if all else fails, there's the National Center for Electronics Recycling (and why not have a bit of a clean-up and get rid of some of those other devices that are no longer needed). They have maps and databases to help you find local recyclers.
Note: You have to be in the right mental state for any kind of clean-out. If you're in a sentimental mood and reminiscing about the past, put it off for another day.