Ubuntu 11.04 Natty Narwhal will get overlay scrollbars but for now the packages that provide this are available in a PPA and further more, you must use "export LIBOVERLAY_SCROLLBAR=1" when launching an application to get overlay scrollbars.

Christian Giordano has announced that Unity will get overlay scrollbars. This means the scrollbars will not be visible until you move your mouse over the side where the scrollbar used to be.