In that video it looks like safari now auto hides the scrollbar when it's not being used (kinda like on the iphone), seems like an odd thing to do, since 20px of screen space isn't as precious on a desktop as it is on a phone.
It's more likely about reducing the amount of clutter on the screen. If you don't need something, why put it on the screen? Apple's minimalist design strikes again.
Edit: Aha, they addressed this: "With the new scrollbars, if all of the user’s pointing devices support both horizontal and vertical touch scrolling, the scrollbars are hidden during normal use. They will appear as an overlay on top of the window's content while the user is scrolling, and remain visible briefly to allow scrollbar dragging."
"Mac OS X Lion introduces overlay scrollbars similar to those in iOS. These scrollbars appear as an overlay on top of the window's content while the user is scrolling and remain visible briefly to allow scrollbar dragging."