we have it on our to do list to include the blog posts for a particular site in a forum search and vice versa as well… however, we do not currently have plans to broaden that for wpmu to include all sub blogs including their forums…

as a two man show (coding wise), we just dont have the resources for wpmu specific type mods… we are on the look for a wpmu developer (thanks Radio for all the support help!) to join the spf team and work on those kind of things…