Gertbuilt brings up some points I have been thinking about. I was told it is mainly the tread pattern as the reason the rear tire is put on backwards, but there is a difference in construction. Mostly the rear tire is built to be more robust, (thus the extra cost over a front of the same size) but I was also told, and someone may be shoveling it fast and deep on me, that there is a slight difference in the way the tire is made - the bias and direction of the cords, the ovelap of the end of the rubber strip that makes the tread before it is vulcanized, depth of tread, stiffness of the sidewalls, etc. The rear has all the acceleration on it, but (according to the MSF) only 30% or less of the braking.
To kep it simple, rear tire on front because it is more stout, backwards for the tread pattern.
As for the car tire, the front does rotate just a little, not nearly as much as on a 2 wheeler. There is a contingent of "Dark Siders" in the Goldwing community that run a car tire on the rear of thier bikes, and these lean over much more than the front of a trike. Seems to me a car tire on the front would last longer and have a larger contact patch.
It's a moot point for me though - can't find a car tire that fits on the front end of the GW1500. I have seen a trike from Europe, can't remember it's name at the moment, with an Earl's type front end and a car tire. Nice setup, but expensive if I remember.