I added the F# keywords to syntaxhighlighter.php and added the F# brush that Elijah Manor put together to the SyntaxHighlighter WordPress plug-in. I am not sure if anyone is still maintaining that project but you can download the zip here (Update 8/3/2020: link is dead) and install it the same way as the original.
To highlight F# code you can now do [sourcecode language=”‘f#’“]code here[/sourcecode]. You can also swap out “˜f#’ for “˜f-sharp’ or “˜fsharp’.