Not sure if it is just because we are doing so much work with the Slingshot, but it seems there are a lot of "interesting" 3 wheelers showing up out there lately, like this project -
This site no longer gets updated as frequently as it used to be, but I think it still has more info on reverse trikes than any other site I'm aware of - http://www.reversetrike.com/.
Update - this site is no longer active and is up for sale.
Dave, the guys collection was stunning. He was like a kid showing me his prizes. His house garage was full and the shop out back was full but they were works in progress. These all ran and were as original as possible.