As noted in this post, if you want Visual Studio just to have an IDE to work with F#, you can get it for free. Now with Visual Studio 2008, Microsoft has released a bare-bones VS, without any language, that can be used with language extensions and plugins, as is the case of F#. This bare-bones VS is called Visual Studio 2008 Shell (integrated mode).
After downloading and installing VS 2008 Shell (it takes a lot of unpacking and installing), you can install F# 22.214.171.124 and voilà, you have a pretty good F# IDE in your box, without having to shell out any cash.
I tested it with F# version 126.96.36.199 in a machine running Windows XP SP2 and no previous version of Visual Studio installed, and it worked. I even compiled and ran some test projects. If it doesn't work at first, take a look at the original post.