I too have not been able to register my console. Bought it off ebay & I think the guy before me had already registered it as every time I put in the serial number it told me that this console had been previously registered. Raised an email to xbox support & got an email back that read:
Thank you for writing Xbox Customer Support! We understand your concern that you want to register your xbox 360 console but it was already registered by the first owner, we do apologize for the inconvinience.After careful review of your issue, we have determined that it is best that you call the Xbox Customer Support number for better assistance;
0800-587-1102 For the hearing impaired (TTY device), you may call 0800-587-1103 Assistance by phone is seven days a week.
Hours of operation:
Monday-Friday: 09:00-22:00
Saturday: 09:00-20:00
Sunday: 09:00-17:00
We apologize for any inconvenience this may have caused you.
Sincerely,
Jill
Xbox Customer Care Team
This came with a reference number. Called them up & I explain the situation to the guy at the other end & he tells me that there is nothing they can do about it for security reasons. I explain to him that I have the original receipt of purchase (from Gamestation in Hull) & I can email him a copy of it. He responds "yeah do that". I say will this help to prove that I have legitimately bought the console & swap registration. He says "Yeah it might do, just email it through" Asked which email address he responded "To customer support" So I said "shall I reply to the one I got from Jill". He said "Yeah that should be fine".
Thinking this guy obviously knows ~@!ck all & does not give a dog's excretion made my blood boil & want to stick the console down his throat & pull it out of his rear....
Valuing my new console I instead ask him very calmly & in a friendly manner if registring the console actually did anyhing and what its purpose was. Guess what, the little american **** didn't know. So I asked him if I had a problem with my 360 & it broke down & I needed M$ to repair it, would it be a problem if it was registered in someone elses name even though the receipt of purchase was in my possession. He said "yeah it might be a problem"

Well, if my memory serves me correctly, Carol @ M$ customer support I suggest you go back to your day job of smoking weed & delivering pizzas cos thats all you good for son!
To all at AV apologies for the rant but if anyone can shed any light on how having a console that is not registered in my name effects my warranty cos if it doesn't I really can't be bothered with the hassle of meeting another Carol. But if it does then I will have to chase up that email that I sent.
Many thanks!
Magusuk