The rules in the USA are the same as they are here. Depending on where you buy from, and the price. Ie. Buy it from Verizon it will more than likely be locked to Verizon.. It won't work on a different carrier, wether it be a US, UK, or anywhere else. T-mobile US is one of the common ones. People buy a US T-Mobile phone, then bring it back to blighty to put their T-Mobile UK sim in... It won't work, and T-Mobile UK won't help you unlock it.
If you are buying from "Best Buy" or Apple, then it's highly likely that it's SIM free, but you have to make sure before you buy. Ask if it is unlocked..