I've been noodling around with F# for a while, partly to get used to the upcoming Functional Programming features of C# 3.0, partly to expand the way I approach programming problems. Now, Somasegar has announced that Microsoft will be making F# a first class language, along with C#, VB.NET, etc. This is very good news, not only for devotees of ML-type languages, but also for FP in general. When something is readily available in Visual Studio, there's more of a likelihood that someone will give it a try*.
And while it's true that other ML implementations for .NET have been around for a while, such as Nemerle, SML, CAML, OCaml and Haskell, the polish that Microsoft will put on F# will make it much easier to use.
Also, unlike some of the above examples, F# is under active development by Microsoft Research in the UK. Don Syme is the leader of the team. F# can be either compiled or scripted like Python, but it has ML's type inference and is type-safe. It generally performs as well as (or better than!) compiled C# code. For an intro and overview of features, check out the project's main page.
* Except for J#. Don't ever try J#.