Personally, if I was to get a replacement for my 2G iPhone, it would prolly ge a 3G not a 3GS, purely due to cost involved.
Don't see why I should pay £750 for a 3GS tbh.
I suppose it doesn't matter how new the phone is, just a matter of the FW, so even if you got the latest phone with the latest FW, you could easily revert the FW to an older one that has been already broken, right?