The reliablity of this trick relies heavily on how you unmount and mount the removable media (firewire in my case) containing your portable Home Folder.
Firstly, you will need a functional Home Folder on the base machine that I'll call INTERNALUSER. The Home Folder on the removable drive I'll call EXTERNALUSER.
To properly unmount the removable drive Home Folder called EXTERNALUSER - logout of EXTERNALUSER, login immediately to INTERNALUSER, and only THEN eject the external drive, logout/sleep/shutdown.
To connect to the Home Folder on the removable Home Folder called EXTERNALDRIVE - firstly login to INTERNALUSER, only THEN mount the external drive, then logout of INTERNALUSER, then login to EXTERNALUSER.
I'm sure Apple could make this easier, but that would seriously undermine the value of their laptops. :)