I have just been looking at the gnome-shell wikis, and I have found some mockups for suggested window management. I think the idea is quite good, could replace the workspaces. Someone has already sent a mail with a mockup similar to these, but this is a bit more detailed... check them out.
PS. maybe the other ideas on the gnome design ideas page for window management are interesting too, I like the one mentioned above the most. Check them out at http://live.gnome.org/GnomeShell/DesignerPlayground