Lion

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-scrollbar {
    -webkit-appearance: none;
    width: 7px;
}
::-webkit-scrollbar-thumb {
    border-radius: 4px;
    background-color: rgba(0,0,0,.5);
    -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.