Getting a bit hacked off with this - no call back from MS, so I called them this morning. At first, they wouldn't even take my call - I pushed all the buttons to get through to XBL support, someone answered the phone and said "go away, we're busy, call back later" and hung up. OK, I'm paraphrasing, but that was the gist of it.
After I'd stopped swearing, I called back. I gave the female who answered the phone the call reference, and asked why they hadn't called back. She got really stroppy and (again, paraphrasing) said "we'll call back when we're ready". I told her the xbox.com forums - as well as others - have plenty of reports of exactly the same problem, swapped-motherboard-and-dvd repairs not connecting to XBL. She said these were "all just rumours"!
I've sent them an email asking for an urgent update, but 8 hours later it's still unanswered. More news as I get it.