It appears the trifolate orange can survive in usda zones down to 6 and that is the zone I am in. So this is probably where its habitat is on its extreme edge as it just gets too cold further up. Sounds like its quite stringent tasting, but I am sure just like with crabapples you can turn just about any fruit into something tasty.
Its always cool to find cold hardy varieties of other plants. Cacti really surprise me in that way too.