I don't know - the fact that this is the only manufacturer I know of that specialises in them would indicate it's not huge, and the kind of people that run Linux are the kind of people that generally want to build their own computers.
If you're thinking of starting one, I would say absolutely avoid the desktop market. We can already build our own custom Linux boxes. What we can't build is a solid, reliable, well integrated high-end laptop. With the MacBook Pro continuing its fall from grace as the go-to developer machine, I'd expect you'd find a ready-made passionate (if niche) market.
