If they'd allow a contract extension so i could move straight from my 3G to the 3GS I might consider it, but they don't.
Those who bought the 3G a year ago have a minimum of 6 months to go on the present contract. The only options you have are to buy your way out of the previous contract or buy a new one and run them in parallel.
For the minor improvements it will provide over the 3G with V3.0 software it's just not worth it. I'll see out my present contract and think about replacing it then.
I have my suspicions that an awful lot of other users will think the same and I don't believe that there are a lot of possible new buyers out there, as opposed to upgraders, especially with the Palm pre about to hit the shops.
So I suspect that, after a few weeks to catch the few who will pay the extra, they'll have to offer a deal to shift the new phones. We'll see.