Eagle-eyed reader, Jack Shaffer, noticed that the screenshot on page 80 of November 2004's Macworld (Rob Griffiths' ever-helpful Mac OS X Hints column) showed a Finder window with double scroll arrows at either end of the scroll bar. "How is this done?" he asks.
Although you can do this via the Terminal, I use Marcel Bresink's free TinkerTool. Just launch TinkerTool, click the General tab, and select the Together at Both Ends option in the Place Scroll Arrows area of the resulting window.