[Bob Williams]

How about a command (or section) in the History menu something like "Recently Closed Tabs", which has as a submenu a list of the last x tabs that were closed? I don't think there's a good way to integrate windows into this without introducing some confusion as to whether a tab should be located under its window or on its own, but even without windows, the user can still get what they needed - which I would guess is usually just one or two tabs in any case.

If you wanted to get a little more fancy, you could make a separate window that lists the tabs, perhaps even with graphical previews.

In any case, doing it this way makes restoration a part of OW's historical browsing record, thus avoiding altogether confusion over in the Undo realm. And restoration, IMHO, fits as well or better into History than Undo. Even if not, it'll make perfect sense to someone the first time they notice it. There's also the benefit that you can let the user control the tab cache via the existing History pane in the prefs.