This is a script which tests to see if scrollbars are available on your browser. They are available on most desktop browsers, but missing from several mobile clients. View source to see how it works, or read the associated blog post.
In my brief testing, this appears to work on a variety of desktop browsers (current Chrome, Safari, Firefox, IE6, 7, 8), the built-in browsers on iOS 3.2 and 4.0, Android 1.6 and a Blackberry. If you find a situation where it doesn't work, or if you can see any way I could improve the script please let me know.