Microsoft has released the code of the F# compiler and core library under the Apache 2.0 licence.
That's the good news. The bad news is that it's just a code drop and so there can be no direct feedback from other programmers into the project - Microsoft still has full control over the development of F#. So it's open source but not collaborative open source.
The compiler was already released under the Microsoft Research Shared Source Licence, which allows users to create new versions as long as they aren't for commercial use. The Apache licence on the other hand does allow commercial use.
Microsoft intends to release new code drops with each Visual Studio service pack.
Get the source code from: http://fsharppowerpack.codeplex.com/