On the official raspberrypi.org forum, "Dom" - a moderator wrote:
I've fudged my board to have your serial number
How do I edit the serial number of a Raspberry Pi?
I'm copying this from this forum thread.
Additionally as posted in the thread. The only reason for changing the serial would be to copy someone else's MP4 licence and use it. As that is the security around the licensing. Your unique serial is linked to the MP4 licence, so even if someone got your licence key, they will be unable to do anything with it (unless they could change the Raspberry Pi's serial number.
UPDATE: To answer the actual question. I'd say that as Dom has the source for the actual low level firmware. I'd imagine that he is really just changing the source code that reads the serial and forcing it to return a different value. I honestly doubt that it was actually changed (on the CPU I mean), more like he changed some of the firmware code to return a different serial. Also apologies to the asker, we all just gave you a "Why? Thats not nice. Your stealing" instead of answering the question. My bad.
A correct answer to this question would actually be a significant breach of the Digital Millenium Copyright Act, which expressly "prohibits circumvention of technological protection measures".
Since the only value this serial number has is the implementation of such a protective measure, this whole question is no different to a "can you please tell me how to hack and bring down xyz website. I swear it's for educational purposes and will not tolerate and suggestion otherwise".