Not in the US. Most of the networks here are mostly incompatible with each other. You buy a Sprint iPhone, you're not going to be able to use it on Virgin, C-Spire, Verizon, etc. In Europe (Spain), all of the networks use GSM technology. I really doubt carriers would be willing to give you a discount on your monthly plan (let's say anywhere from $40-$50) if you bring your own device. If they decide to pay for the subsidy then they know that 1) You will be obliged to stay with them for 24 months and 2) They've paid off the subsidy and are getting *profits* from the device if you haven't upgraded in 2 years... which many people end up doing.
I really doubt they'll do something like that in the USA where the iPhone is so popular (unlike in Spain or Greece). I know soooo many people with iPhones, but I don't think more than 2-3 would buy them at full price.