I added a box-shadow to a section of a page recently to give it the same shadow border effect that is seen on Mac OS X apps. It looked great, but I noticed that
box-shadow
This has been fixed in webkit as of two days ago. :)
https://bugs.webkit.org/show_bug.cgi?id=22102
You can pick up a chromium nightly to try it out.
I looked in FF3.6 and FF4 and don't see terrible scroll performance there, so it might be addressed there as well.