Because the first iPhone was unsubsidized, you just need to extend your contract by two years and then you end up with a new iPhone 3G and you have an old iPhone you can play with. iPhone 3G FAQ: How to replace your original iPhone with new iPhone 3G – iPhone Hacks
iPhone owners in the US:
If you are in the US then you don’t need your original SIM as the new iPhone 3G includes a SIM. Your original iPhone account information will be transferred to iPhone 3G’s SIM at the time of purchase.
The 8GB iPhone 3G model will be available for $199 while the 16GB model will be available for $299 pricing for the following customers:
* iPhone customers who purchased before July 11
* Customers activating a new line with AT&T
* Current AT&T customers who are eligible, at the time of purchase, for an upgrade discount
(you can check your eligibility to upgrade here)