The newest design for these machines and their counterparts (for example, regular laptops running Windows-based software, like I/Mobile from Intergraph) is called "Mesh" networks. It's a kind of a peer-to-peer network sharing operation. Kind of like having WiFi hotspots AND ad-hoc (computer to computer) networking at the same time. Motorola had a demo of this displayed at a conference I was at in early October - imagine a network of antennae on light standards, traffic signals, etc., and in each vehicle as well. Vehicles and other terminals (i.e. a police officer with a Palm-like handheld) connect to whatever tower is nearest them and are 'handed off' to other towers as the need arises, much like a cellular network.
The neat part is when you have a unit that is far, far away from the established wired network. Let's say a fire crew is at a hazmat incident 15 miles up the road from their home city and needs their MDT for entering call info, or getting MSDS sheets, etc. The demo seen at the conference postulates that you could use vehicles as extenders every few miles, so a police car could drive out from the city to a point where he is still getting decent MDT signal, then a battalion chief could drive further out from the police car to his coverage limit, and so on, until the gap between the city's network and the incident was spanned.
Granted, you're not going to want to tie up a whole bunch of vehicles and personnel on a regular basis to do that, but the idea is just being developed now. Perhaps they will have self-contained deployable 'extenders' you can just toss on the side of the road for that purpose.
As for the (il)legality of scanning these things, that's most likely only in the United States. I accept that I may be the only non-American participating in this thread so far, but just thought I'd point that fact out.
My agency is currently using regular (well, ruggedized) notebook computers in its vehicles. The communications are running on a data network called CDPD (Cellular Digital Packet Data, if I recall correctly) which is being phased out. They're looking to have the replacement be able to handle not only the MDT and AVL (Automatic Vehicle Locator) traffic it covers now, but also our text/alpha paging and other non-voice comms (Internet/intranet in the trucks, anyone?).
