I am keeping mine and giving it to the wife and kids. Also, as far as I read you can reuse the screen if you shatter the 3gs.
I have to think the market is going to be saturated with used 3 g phones.
MBP 2.8 GHz i7, 4GB Ram, GT 330M 512MB
iMac 24" 2.8 GHz, 4GB Ram
Mac Mini 2.0 GHz, 4GB Ram HTPC
Black iPhone 4 32 GB