While they do not invalidate the premise of a windowing system, tabbed browseing seems like a bad UI convention. Mainly because tabs, to me atleast, should be used to show different views, or different controls for the same data.
Like in your System Prefs example, the different items are not all in different tabs, but within the item, there are tabs to show different groups of controls for that item.
Tabbed browsing seems to me to break that convention. Each tab can have a totaly seperate page on it, and they don't have to be related, save for the fact that you opened them. Just bad UI if you ask me.
But I won't stop anyone from asking for it, or including it in a browser, if you want it fine. Just don't call it good UI.