I can't see how this might be implemented without making the UI complex and messy. Anyone care to share a mockup or description of how they'd want to see it presented?
I usually use new windows for 'topics' and tabs for pages on those topics (eg right now I have a window full of tabs browsing the Omni forums for Spaces support in OW, and another window full of tabs about the new iPhone SDK). I also have a 'higher level' arrangement (types of topics?) which I arrange using Spaces; all my general interest windows in one space, all my shopping / finance windows in another, all my software development windows in another.
And then, of course, there's workspaces (I used to use these for my 'higher-level' organisation, now replaced by Spaces).
So I'm struggling to see what this feature would add to OW that isn't already covered by tabs, windows, workspaces and Spaces. Anyone care to give me an example of how they might use it if it were implemented?