BSD development work on a Mac?

Although I've been a Windows-with-a-little-Linux developer for years, I'm
always exploring new areas in which I might train. Since the OSX operating
systems seems to be winning a high degree of critical praise for Apple, has
anyone here tried using OSX Macs to do Unix development work? Do you see
much of a market for this sort of thing?

After all, the BSD core of OSX appears to be a full implementation of Unix,
with a GUI that seems to work far better than any of the other Unix graphic
shells on the market today.

Alan Gore