I just finished converting my Downtube folding bike into a recumbent with a kit from Cruzbike.com. The kit was previously installed on my Specialized mtb, but it turned out too heavy. So, a few hours in the garage, and ta-da! A folding recumbent!
It's probably about 27 lbs, feels very light and nimble and should take the hills around here pretty well. The fold is a bit compromised right now because of the hinge in the top tube, but I'll fix that with a custom bracket eventually. Should be fun to ride!
The bike used to look like this:
Now it looks like this: