I've done this, but the disk I used may have been a pack in upgrade disc not the restore disk. Can't remember... should work either way, even if you have to use one of those 3rd party package installers to do it.
As far as legality goes, you'll be in a grey area that depends on the exact terms of the EULA. Morally, though, you'll be pretty OK assuming you are the only user of both machines. Some EULAs indicate the software is licensed to ONE PERSON on whatever computer they use, some say one machine, no matter how many users are on it. I don't know what Apple's say. Probably one machine, since they tout the multi user features of OSX.
Hope that helps.