Microsoft has included F# in VS 2010 Beta 1 and has released a corresponding CTP update for VS 2008. The latest binaries, version 1.9.6.16, are not compatible with previous ones, v. 1.9.6.2, meaning all previous code needs to be recompiled.
Microsoft has not only included F# in Visual Studio as promised in 2007 by adding it to VS 2010 Beta 1 with support for .NET 4.0, but it has also created a VS 2008 CTP update for those who don’t want to use a beta but a more stable IDE along with .NET 2.0-3.5.
Don Syme’s detailed release notes mention a binary incompatibility:
F# binaries compiled with this new release are incompatible with binaries from previous releases, so must be recompiled. Binary compatibility for F# is an aim of the RTM release of Visual Studio 2010.
Another significant breaking change is using the lightweight syntax (#light) by default. That makes the code whitespace-significant by default. The MSDN Walkthrough contains examples that do contain illegal whitespaces which need to be removed.
Useful links: Release Notes, InfoQ interview: Don Syme Answering Questions on F#, C#, Haskell and Scala, F# Content on InfoQ, PDC Presentation on F# by Luca Bolognese (beginners), MSDN Walkthrough of F# in Visual Studio 2010, MSDN Visual F# Documentation, MSDN F# Code Gallery.