Hmmm... Would I be able to use the built-in Macbook keyboard and have the monitor turned off to run only the ACD? I think I couldn't (or didn't know how to) do that when I connected my Macbook to an older 17" CRT monitor. I could either run it in mirror mode or span across both.
First, I wouldn't use mirroring, as it will display at the same resolution as the Macbook screen (and would look lousy).
It is a bit of a process, but if you do the following it will work.
1. hook up external display to the Macbook (extended desktop)
2. close the MacBook display (sleep computer).
3. Awaken by using external mouse or keyboard (well... hopefully you have an external mouse).
4. When awake, open the Macbook to use keyboard (display will stay asleep)
Although I highly recommend using an external keyboard & mouse