Starting from Mac OS X Lion, scrollbars have been transient. That is, they disappear when your mouse stops moving. Some sites, like Facebook, even emulate this transient scrollbar for Windows users:
I had the opportunity to interview a design/usability candidate. I asked the candidate to use our website and critique it. I only had a Lion laptop, so she had to critique using Lion. One of the panes on our site was scrollable, but she did not realize it until I mentioned it to her. This design/usability candidate is a Mac user and the lead designer at a very reputable website startup and even she couldn't discover the Lion scrollbar.
So here's the million dollar question: