It's only an element of the EULA, and as long as you're no longer using the copy of XP on your laptop (say you've put Ubuntu on it or something) there's nothing to prevent you from installing it on your Mac*
After all, there are no EULAs preventing you from removing the hard disk from the laptop and using it elsewhere, is there? A lot of EULAs are practically unenforceable, as is for example Apple's stipulation that you can only run their OS on their hardware. This is just such a case. Ultimately you can demonstrate ownership of the license, and that it is being used only on one machine.
So, once again assuming you don't have XP on your laptop anymore, give it a try.
*Unless the OS installation on the disc is locked to the OEM Vendor ID in the BIOS. This is done by very, very few manufacturers but I do remember Gateway being such a vendor when I had Windows 2000.