I think I am going to keep my old one after I get the new. Main reason is that my current one is jailbroken and there are a lot of programs that I really enjoy. Once the new one either has a good selection of programs available through iTunes, or has a way to be jailbroken, then I will abandon my old phone and give it to one of my sons to use as an iPod.
I decided to sell mine and wait for a new one. It is used, not jailbroken or unlocked, and in good condition. I put it on eBay around 9PM with a buy it now of $385. It sold and I had the paypal money before 5AM the next day! I guess I should have asked for more, but that was only $15 less than I paid for it. Now I'm waiting eagerly for the new one!
BTW, AT&T let me remove the iPhone data plan from that number. I gave the number to my son who put it on a phone he already had.