(Digression: apparently it is possible to replace the firmware inside most mobile phones with hacked versions, much in the way that car hackers can reprogram their BMWs' engine controllers; how much effort is required, I do not know. I can think of some neat (and benign) applications for this; having a phone display the relative strengths of several nearby base stations would be somewhat cool, in a somewhat trainspotterish sort of way.)
Though, just from following a search query, it seems that unlocking stolen phones and changing serial numbers is so trivially easy that any petty thief with a PC could do it. Which is rather stupid on the part of whoever designed the system.