The problem with allowing the customisation of scrollbars is that most people are going to make them worse.
* Personally I use the scroll wheel, but I know a lot of people who don't.
now along the same lines, when are we gonna get clipboard access
iTunes is the lone offender. Those weird flat scrollbars were introduced a couple of years ago -- maybe iTunes 7.0. Perhaps Apple's justification for the change was that they wanted iTunes to look the same on both Mac and Windows.
(Edit -- it worth mentioning that 10.7 Lion will introduce iOS-style scrollbars to the Mac, so we can probably look forward to more freeform scrolling experimentation by app developers as well.)
While I don't care about the styling, every single linked example on that page doesn't work with space or the pagedown key. I wish there was a bit more polish here...
If you're going to do it (but please, don't), do it like this. Not with some crummy Flash applet. I'm talking to you, Vimeo.
The web has grown up since IE5 (where you could merely color the scrollbar) and graphic designers will design some proper ones fitting with the whole site design, while a bad scrollbar design will just be an element of an equally bad site otherwise.