Go to MacUpdate.com and search for "Spanning Doctor". They have it. A word of caution about "Clamshell" mode (when you close the lid and just use the iBook to power an external monitor), there may be heat issues. The documentation even warns about that. A good portion of the heat excapes via the keyboard. By closing the lid, you effictively trap it.
You might consider just enabling the spanning, and then dimming your iBook monitor to the point of it going dark as an alternative.