I think OCaml would be a big win if you have your software designed all up front, and need to verify that it works correctly, e.g., avionics software. Unfortunately, avionics software development seems to be moving toward C/C++...
"In Nov. 2003, ASTRÉE was able to prove completely automatically the absence of any RTE in the primary flight control software of the Airbus A340 fly-by-wire system, a program of 132,000 lines of C. [...] From Jan. 2004 on, ASTRÉE was extended to analyze the electric flight control codes then in development and test for the A380 series."
Go&learn C++. It blows my mind how often people confuse C++ with "C with classes" in 2008. Granted, C++ isn't as sexy as OCaml, but it's still C plus some rather nice extentions that (finally) got trully portable and work as advertised.
Powerful templates plus multiple inheritance give you a very impressive weapon to play with. C# and Java don't even come close.
And please... 99% of software on Windows, Mac OS X and Linux are built using the same stuff: C/C++/ObjectiveC.
I think OCaml would be a big win if you have your software designed all up front, and need to verify that it works correctly, e.g., avionics software. Unfortunately, avionics software development seems to be moving toward C/C++...