If you bought "device only" it is unlocked. Which means you bought it at full price. The iPhone 5 also has to be brought "device only" to be unlocked. Otherwise all AT&T iPhones that is acquired under contract is locked to AT&T.
It is locked, but you can use a Straight Talk SIM on it without unlocking it as far as I know... I know for sure you can on the iPhone 5, I just did it...