I considered the idea of building one (9Zero7 frame probably) but in the meantime they seem to have been breeding around here (On-Ones mostly) and now there's a whole scutch of people riding them. Sheep.........
Not being someone who likes to be seen to be jumping on the latest bandwagon, I don't think I'll bother with one at the minute.
They'd be bloody hard work as a singlespeed too......