Full price U.S. iPhones are not factory unlocked, they are just "no contract." However, they are still carrier specific. You can't take a Verizon iPhone to Sprint or vice versa or an ATT to T-Mob. You'd have to go to Europe to find a factory unlocked iPhone.
I would have assumed that the iPhone 4S is available as an Unlocked device since the iPhone 4 has been available at Apple Stores and Apple.com/store at $649 for a 16GB version fully Unlocked.
Calling Apple now...