I just saw the new UI in NetNewsWire 2.0, particularly where the tabs are above the Browser buttons (back/forward/address/etc). Since these UI elements are dependent on the tab, they should be below them.
Sans OmniWeb, AdiumX, and Colloquy; they aren’t yet outside the window completely, but it’s getting there. Ultimately, though, tabs should be part of the window manager, and should be application agnostic, enabling tabs from different applications to be grouped together. Grouped Windows, anyone?