I've seen this pop up a few times and each thread says "if you have this issue, start your own thread."
My brother and I bought a first generation Xbox 360 and made a gamertag using my Live account. He got married and bought his share of the Xbox off of me and changed the email address to his own. I then bought my own '360 and made another gamertag under my original Live account again, thinking all traces of the old gamertag were migrated. He then sold his Xbox, so he no longer has a way to sign into his account.
We are now trying to restore his Gamertag/profile to my Xbox, but while he logs in with his email address the recovery stuff routes through mine (but doesn't show up when he tries to reset his password.) So unless we can migrate the rest of his account over to his own Live email address, we can't get his account back.