If you have two different systems, it is possible to share the same license (not legal however). It requires you to make a HFS+ partition on an external HD, use Disk Utility to restore your already installed version of OS X to the external partition, then booting from that partition from the other computer and restoring the external partition to the computer. Voila. It's very helpful for it-supporters and the like. Funny how Vista completely rejected this nifty feature so now you need to spend 2 hours installing everything from scratch. Imagine doing that to 200 new PCs. Ouch.