> As far as I know, all require propitiatory maps. Garmin even requires the purchase of a CD for
> each unit. If you have 3 units all using the same map, you have to buy 3 licenses. Not sure if this
> is for all units, but be sure to check it out.

Here in the UK Garmin let each CD be registered to 2 units. I gather you can unregister an old unit and transfer the licence to a new one if you contact them, but I've not tried that myself yet.