So here's the second update.
This fellow here http://brokenxblm.blogspot.com
Is also having issues with Xbox Live.
I figure since his problem is simila why not plug it.
To summarise what's happening for him and many other Xbox Live users is the following
Xbox live identify you by both mac (serial#?) address and gamertag.
When your Xbox 360 breaks down (and sorry to troll but it WILL break down) your purchased legimately Xbox Live content doesn't work properly on your newly replaced Xbox 360.
An Easy solution to this would (at the database level) simply be to get your gamertag, associate it with a new serial number (link / bind - your choice or terminology) and off you go- Xbox Live thinks you're the real deal (as it should, you're a genuine paying customer)
Sadly, at the moment - not the case.
Ultimately this i s not the same as my problem at all but it is a case of Microsoft having users have to cock around for a while to fix things where updating a single field on a table should (theoretically) fix their problem.
Go check out his site as he's written it up far better than I can.
I wish I had the same problems - but since I'm such a fuss/whiner - I refuse to sign up to Xbox Live and buy an X360 until I can get the gamertag I used to have. (preferably under the passport.net id I'd prefer to use!)
Time will tell