Your actually quite wrong, just got a Motorola 4100 brand new and it works....so their ya go.
The only thing that stops is the way the mac address is laid out, the start has to be in a certain order i guess, from the info i got when a rang NTL multiple times. And the different models must have different start sequences for the mac address.
As long as it is one of the models the NTL let you use previously before they went to the cheapo types it will work.
But i can definatly say you CAN buy a motorola 4100 and use it with NTL, it DOES work.
Ya learn something new everyday =)