It's great that the new Safari version includes tabbed browsing. It's even better if Apple builds a similar feature at the OS level to benefit all applications. Multiple windows in any application could be optionally grouped to make them more manageable. One easy way to do this is by dragging one window title onto another to create a single window with tabs.
This is similar to Photoshop's tabbed palettes which can be rearranged by dragging individual palettes in and out of groups in the same way. Try this in Photoshop to see how well it works.