I think this is a moot point, as we've seen time and time again using technologies such as Clojure/OCaml/Haskell/Etc is a boon for hiring great talent, not the other way around.
An excellent point. Definitely something we should all keep in mind when promoting cutting edge tech choices.
That said, I don't think the matter is quite that straightforward where I come from; I would imagine Clojure dev head count in the whole of Finland is in the low double digits. If I were making business decisions involving future staffing, I would not feel confident I could replace the (hypothetical) only in-house Clojure dev in this job market.
But what's the number of good developers? I think the point is that good developers can learn whatever language they need. You don't need to hire Clojure developers.
Yes, thats something that some people don't seen to get. Good developers not only can learn new languages as needed, but they will learn new languages and idioms with time, else they will quickly become obsolete.
I'm also a pretty big proponent of distributed teams :)
That said, as others have noted, if you're hiring quality people, there's no reason they can't learn Clojure on the job. Plus if you make an investment in your people they're more likely to be invested in you ;)
http://janestreet.com/minsky_weeks-jfp_18.pdf See part 4 "Personnel" for Jane Street's experience hiring OCaml devs.