A few do, but the vast majority do not; the cost differential is quite large. A unlocked phone will generally cost 2-3X what you would pay for a locked/subsidized phone, and since the cost to get service from any carrier is generally not going to cover the difference in price over the lifetime of the phone it is not a good option. One additional factor to consider with the US market is that we do not have a single cellphone standard; CDMA phone users can choose between Sprint or Verizon and GSM phone users can choose between AT&T or T-Mobile (there are minor prepaid players as well, but prepay and smartphones will not be a good mix until the providers figure out how to deal with the voice vs. data issue...)

It is possible at some cell phone stores to purchase an unlocked phone and renew a contract at the same time for a discount on the hardware. Discounts can range from $50-$200 depending on the phone and renewed plan.

Unlocked phones are available at many of the big electronics retailers (best buy, frys, etc) and there are myriad cell phone stores that specialize in unlocked/imported sets.