The device protocol is supported by the EZWeb (Openwave) browser on handsets used for the KDDI/Au service in Japan. A specification of sorts for the device protocol is available online at the Au EZfactory site.
The protocol facilitates "location aware" Web applications. Specifically, it enables a remote Web application to query a mobile device for its position/location information (exact latitude and longitude) and to then pass on that position information to a remote Web application that then uses the data in some way (to display a map or other location-specific content).
Also available is a simple "Where Am I Now?" example of how to use the device protocol to display a map. To try that example, you need to access the URL http://sideshowbarker.net/gps/ from the EZWeb browser on an Au handset.