Don Syme in his SPLASH talk says that F# is NOT intended to be a replacement for C# even though it has the general capabilities. He goes on to say that there are areas where F# makes no sense in using, but doesn't expand on the thesis.
In what areas might the use of F# be more appropriate than C#?
Good question. I'd say that there are zero language reasons and many unfortunate skill-set, aptitude, and attitude reasons on the part of developers, managers, and customers.
Well, at the risk of stating the obvious, F# is first and foremost a functional programming language and OOP programming in F# can be a pain in the neck. So if you're working with a problem that is best expressed with OOP I imagine using C# does make more sense.
Mutually recursive types and explicit implementation of interfaces are the first examples off the top of my head of why OOP in F# can be cumbersome.
An (often cited) example of "problem that is best expressed with OOP" is creating a UI library. You have many widgets, which encapsulate their own state and you want to ask them to do a few things like "draw yourself" polymorphically (is that even a word?)