If we assume the battle is between C# and F#, I do not think F# will win over C# within 2 years for the following reasons:
The features of F# that C# does not have are not features people have been missing. For instance, I think Seq.map, Seq.iter, Seq.fold and friends are great, but I don't see a majority of developers switching from foreach to these constructs.
The performance benefits of multicores are irrelevant to most of the existing programs, as only few programs are cpu-bound. For those programs where performance really is important, (e.g. video games), C++ will remain predominant, at least for the 2 years to come. It's not that hard to use threads in C++, assuming one avoids side-effects (which you can decide to do even in C++). Isn't that what Google is doing?
For F# to become really big, I think it has to become one of the main languages used in teaching, the way Java has been. This is actually quite likely, seeing how the academic world is fond of functional languages. Should that happen, I don't think the effects will become visible before 5 years.
