Switching SIM cards from different carriers no longer works on an unlocked iphone4

I am no longer able to switch SIM cards in my Iphone 4 which used to work before updating to iSO 6.0.

Why is APPLE now locking the Iphones to a network when I specifically bought an unlocked iphone 4??

Can anybody help me with this problem rather then I having to always re-activate the phone when I want to switch the SIM card having to use a PC and your Itunes account for re-activation??

Thanks,

MAX