This is the result of the first afternoon spent hacking on the ayatana overlay scrollbars for the next release, Ubuntu Oneiric Ocelot. The visuals were updated and the behavior next to the screen edge was changed.
Visuals: the thumb features an orange border which becomes grey when the overlay is detached from the thumb (like when clicking pageup/down buttons, or scrolling the wheel on the thumb).
Fitts’s law: the thumb is bounded within the screen. With this behavior the ayatana overlay scrollbars can be even more usable than native scrollbars, because Fitts’s law is preserved even when the scrollbar is not touching the edge of the screen (few examples: gedit or others apps where the scrollbars are not touching the screen, or on any windows not maximized but close to the screen width). Also, the thumb is now placed over the overlay, that means you don’t have to aim to the overlay and then move right, cause you’ll immediately catch the thumb. A real pleasure to use.