The Datacolor License Agreement for Spyder3 allows you to install and use the software on one or more computers at a single physical location. So you can install the Mac and Windows version (the software serial # is the same for both) on any/all of your desktop systems at a single location and move the sensor around between the systems to calibrate them.
In addition to this, you can install and use it on up to five additional portable computers (laptops).
Senior Software Developer, Digital Color Solutions
—Just for my own reference. Bought a Spyder3Elite today, upgrading from my old Spyder2Pro.