I remember a few years ago hearing something that cell carriers could not tie equippment to service. Back then however everyone was using different standards and the phones were not compatible.
I think now they are required to unlock your phone if you requrest it. Will need to double check the FCC rules on that though.
I know with Tmobile all you need to do to get them to unlock your phone is say "I'm going to Europe for a few months and will need to use the PrePaid SIMs while in country"
The store here used to unlock it no problem. Been a while though
Wow I'm just full of outdate information for this thread.
Time for bed.