FCP doesn't really like to scroll the timeline while you playback footage. And while I wish there was an OPTION for it, I'm okay with it not scrolling at all. It really does save some CPU when you're doing HD muiltclip editing and other CPU intensive tasks as well as increase your bandwidth usage as FCP pulls thumbnails from the footage on the drives to display in the Timeline. We'd probably see a lot more dropped frames if the Timeline scrolled all the time. At least in FCS2 on Leopard.
However, if you have a Mighty Mouse you can scroll with it's trackball while you're playing footage in the Timeline but this will eat up some CPU cycles as well as use a little more bandwidth from your hard drives. But, if a client insists that they see where the playback bar is while you're editing and playing edits, keep your fingers near the Shift and PageDown keys. Shift + PageDown will advance the timeline one "screen" at a time during playback.