I'm not sure if it's the public IP, but I remember hearing early on that they set up the system so that it only has two IP leases. So if they both get used up (eg. swapping 2wires, or possibly even just hitting the red reset button on the original unit) then the system locks it out for 24 hours. I think this is a measure to protect against unauthorized use of the system.
I don't know the exact details, so don't quote me. I just know that it's picky that way. Say if I have a customer who is transferring their Ultimate service (weird to have that already) and they carry their equipment, if the 2wire was unplugged less that 24 hours ago, it's a given that it will get locked out if I sync it up at the new place. So I think installers are just swapping them regardless now.