Can I set Lion scrollbars to always be off? (Except when a mouse is plugged in)
The option you mention in System Preferences does exactly what you say (or its behavior was changed since your question).



The first option, "Automatically based on input device" does what you want (I think, but I may have misunderstood your question). When you have a mouse plugged in, it shows the scrollbars. When you unplug your mouse, it will revert to the default of only showing scrollbars when you are scrolling.

I hope this answered your question. Please let me know if it doesn't, as I probably misunderstood what you are asking.