Having two monitors has been difficult for Wacom tablet users. For the past year I have just been turning off the laptop screen because the support for two screens was just not available. I tried creating an apple script that would allow me to switch which monitor I wanted to use but in reality, although it did work, it was slow and more frustrating then useful.
The complaints date back to 2002 about this lack of multiple monitor support but this has finally been fixed in a driver update recently released. I am now using "Driver 6.05-3 (RC) for OS X v.10.3.9 & later (PPC & Intel) — Nov. 29, 2007" and it works great. It's exactly what I needed.
It is very simple to use. Just map any of the express keys to "Display Toggle" (option only appears when both monitors are plugged in) and the pen cursor quickly switches screens and maintains a mapped ratio of 1:1. Perfect!
Disclaimer: This driver update is only available to what Wacom defines as their pro products. This feature will only work on Intuos3 and Cintiq. It will not work on either the Bamboo or Graphire models.
If you own an Intuos3 and have more then one display, download the latest drivers from Wacom. It will make your secondary display much more useful, or should I say accessible via the tablet.