If you buy a sim/contract at the same time as the handset in an Apple store it'll lock to that sim card as it's processed through the system.
However if you decline buying/taking a sim with the purchase, it'll be an completely unlocked phone.
So basically don't take a sim card/sim swap/contract when buying an iPhone from the Apple store and you'll have a sim free unlocked phone.
I have just got back from the apple store in brum, bought a 64GB space grey 5s, fully unlocked, no carrier what so ever, put my 3 sim in, works fine, but my orange sim in and tells me its not compatible with this sim, so now my phone is locked to 3?

(even if ios7 is a bit weird)
