The Leica Passport IS transferrable, at least in the UK.
I've just bought a used M7 with passport until 2009 (I think). I phoned Leica UK to confirm the procedure for transfer - you have to send them your details and copy of the original Passport. I was asked whether or not it was a private sale (it was) "just out of interest" and assured that this makes no difference. The actual "Passport" is a small black card with the original owners details printed and then, much like UK Vehicle V5(?) form, there is a space specifically for a new owners details.
Some UK shops, incl. those that aren't authorised dealers, add a premium for bodies / lenses with Passport cover - ApertureUK had an MP (or might have been an M7) with passport cover until 2010 that was priced higher than equivalent bodies they had for sale. I know I paid a little bit extra to cover accidental damage et al.
If this isn't the case then I've been lied to twice - once by the buyer, once by Leica UK HQ.
<hunts for number of Leica for double-confirmation... />.
Joe