Apple Inc. is selling the phones on its website and at stores for $649 and $749, depending on the amount of memory.
Buyers would also have to buy a carrier's SIM card.
The unlocked phones are identical to the AT&T iPhone4, but do not require a two-year contract.
Currently, the only other compatible national U.S. carrier is T-Mobile. But T-Mobile's "3G" wireless high-speed data network is not compatible with the iPhone.
Many overseas carriers, however, are fully compatible with the phone, so international travelers can switch out their U.S. SIM card with one from the local country to avoid AT&T's international roaming fees.
Apple already sells unlocked iPhones in all other countries where the phone is available, according to Apple spokeswoman Natalie Harris.
Hackers have also made a sport of unlocking iPhones, freeing them up for resale and use in other countries.
The Associated Press contributed to this story