Don't know if I'm helping but I have been thinking about this too.
Have you checked out Cinelerra? I don't think it's free but it's what Broadcast2000 turned into, and I think you should be able to turn up a copy of that -- e.g. Broadcast2000 came with my Suse 9.1.
(I think Broadcast2000 is what FilmGimp turned into.)
It's pretty serious software, but recommended power for Cinelerra is dual Opteron and/or some spare CPU power to make a renderfarm.
I have not had a chance to try out Broadcast2000. I am wondering whether I can get around the rendering muscle problem by only working on short pieces.
As far as I can make out there is nothing (open source) that gets near the standard of, e.g., Final Cut.
The Linux Movies Group keeps a list of related software:
Last time I looked there were a few dead links/dead projects there.
I have not tried MainActor since upgrading my system, but previously it was quite crashy.
I am also planning to look at Jahshaka: http://www.jahshaka.org
and Zweistein: http://www.zs4.net/
Eagerly looking forward to other poster's recommendations...