So here it is:
Free options will continue to exist, most likely through the VS shell. For example, we'll be continuing with our VS2008 plugin until the plans around a VS2010 shell finalize
We're unlikely to have a dedicated VS Express version for a while.
Good to know!
(I am not in a position to use F# at work since it is a Java shop, but it is comforting to know that I can use it for recreational coding without investing in a Visual Studio license.)