I wonder how many people actually write Haskell,
principally or exclusively, at work?

I suppose you're implying non-academic jobs by that statement, but most of the people in my research group develop programs in Haskell on a daily basis. You'll find a number of libraries on Hackage from us.

http://www.cs.uu.nl/staff/cur/IDX/sds.html

As a shameless plug, I will also add that we have a great master's program in which you can get your fill of Haskell and compilers, among other things.

http://www.cs.uu.nl/wiki/Master/

As second (but related) shameless plug, we also have a two-week-long summer school which is an excellent way to jump-start the above master's program or to get quickly up to speed on Haskell for business or pleasure. The course is in August, and the deadline is May 1.

http://www.utrechtsummerschool.nl/index.php?type=courses&code=H9

Regards,
Sean