That is correct. Typically licensing for virtual machines is identical to physical machines - ie pay for multiple companies. If they tie the license to hardware hashes it uses the virtual drivers. However, one main goal of virtual machines is to abstract this so, in most cases, once activated, if you copy the virtual machine to another physical one the software will most likely remain activated. You'd want to test to make sure if you're using this for a mission critical backup or whatnot.
Not having used anything other than the virtual XP built into Win7, I am curious what hoops you need to jump through to activate the MS operating system. Is it activating itself based on pseudo hardware settings in the VM instead of the hardware of the device you were running the VM on at the time you activated the OS?