In time for summer, Apple has shed all the cloaks shielding it's carriers and now offering unlocked iPhones in the USA, directly from Apple store. Just like it did last year in Europe. Now you don't have to clog through Craigslist for an unlocked iPhone 4. The 16 GB iPhone 4 will cost US$649, while the 32 GB model will retail for $749 in either black or white color.
It is the GSM version of the phone and that means you can use it on AT&T network in the USA. The other GSM carrier T-Mobile is not compatible with iPhone.