It's possible that the market price could stay the same or even go up if Apple makes 3.0 super secure and unable to unlock. Then all the people who want the phone who are on other networks will have to get the 3G pre 3.0.
Assuming O2 will allow me to upgrade to a new iPhone then I'll probably give the current one to my girlfriend. She's got a really old-school Motorola and could do with a newer phone
I was planning on using the wife's first gen as an exclusive apple tv remote, giver her my 3g and getting the new one for myself. But she sold her first gen for a good ammount so she jsut bought a 3g. I think I will probably sell mine to get the new phone but I will see, I have an upgrade due in December.