We use Google Prettify
http://code.google.com/p/google-code-prettify/
with an extension to hint at certain supported languages based on the question tags:
http://meta.stackoverflow.com/questions/72082/changes-to-syntax-highlighting
what language, that Google Prettify supports, can we use for f# then?
(and note that diamond mods can change this setting on the tag info page..)