All you need to learn if you want to become a professional .NET developer is C# and VB.NET. However, there are several uncommon .NET languages in development, which includes F#, Delphi.NET, Boo, IronPython, IronRuby, Nemerle, A#, and about two dozen others.
I've seen a few job postings for F# programmers, and I worked for a company which used Delphi.NET in a small number of projects. However, what about the other languages? As near as I can tell, there is no commercial demand for these uncommon languages, the userbase consists of a tiny community of hobbyists, and several of these languages are no longer being actively developed.
This is a somewhat open-ended question, but is there any point to learning one of the uncommon .NET languages if there is no commercial demand for them? Or should the languages be considered strictly toy languages and unsuitable for any "real" development?