Has anyone tried VirtualTourGuide.net? It appears to be similar to Gypsy Guide which I used in Jasper and Banff National Parks which was great. It is a GPS unit integrated with points of interest files that are broadcast over the car radio. When you approach a turnout, photospot, turnoff or place of interest you hear the background, historical information and interesting stories relating to the site and surroundings.
The Gypsy Guide unit I first rented had bugs (I had to reboot it every time I turned off the ignition) but the exchanged unit worked like a charm. Well worth the $150 for 4 days.
I expect to visit Yellowstone and would like to hear of anyone's experience with the VirtualTourGuide