It depends on the baseband version if you can unlock or not. If you've updated to 4.2.1 without preserving you baseband then you can't unlock.
Double check your baseband version by going into the settings > general > about > modem firmware. If it's anything other than 1.59.00 then you can't unlock...
some times carriers will factory unlock it for you .. you can ask them about it.. unless you went and bought it from another country or carrier that you never used..