After seeing Opera's tab grouping experience I got to thinking about what benefit/drawbacks there are to such a lightweight tab stacking system. I decided that it's actually quite useful, provided that it exists in a larger tab grouping infrastructure... Tab Candy.
Here we've got some tabs that have spawned from my gmail that I don't want to deal with, but that I also don't want to "promote" to their own group.
Dragging a tab between other tabs would rearrange as it currently does, but dropping on another tab would stack them.
For this particular use case it would also be nice if we could have a command (right click menu?) that says "grab all tabs that spawned from this source and stack them".
Now we've stacked the tabs up, changing the x on the tab to a +, which would expand the stack back out.
Obviously this stack should be reflected in the Panorama UI. The initial state of our tabbed browser would look like this in Panorama:
After we stack up tabs (which should be possible from either the tabbed browser or from the Panorama UI) the Panorama UI should have an actual stack reflected in it: