i'm doing exactly that.
i setup apache to mod_rewrite and proxy to an iis site.
I also used
to rewrite the action="" of the form element to nothing.. the browser then postbacks to the same url as in the browser which apache interprets to the same url for iis.. so that the postback mechanism works.
luckly my setup was easy as i have open suse as my base with windows 2003 server running inside vmware.