Ipaqs' look pretty popular. I've got the Destinator version, plugs in the bottom of the Ipaq rather than being a jacket, so the Ipaq retains its slender lines but is useless as a stand alone unit i.e. out of the car.
Cost, £320 for the full Europe version, oh then there's the cost of the Ipaq (£400-500) and £80 for SD memory card needed to store the map data
http://www.destinator1.com/
There's also an addon, available today for the TomTom system and soon for the Navman and Destinator systems, from Trafcam which integrates with the sta nav using the GPS and it database of fixed camera sites to warn you of their presence and if you're exceeding the speed limit
http://www.trafcam.com/
Graeme