Personally, I prefer the hidden scrollbar, but in case you really need it, you can just overwrite the default and force the srollbar back like this:
-webkit-box-shadow: 0 0 1px rgba(255,255,255,.5);
See the demo.
PS. And as an added bonus, it gets rid of the ugly scroll-track in Chrome and makes it look more consistent.
PSS. If you only want it on Lion, here with a JS sniffing script added.