As per this article:

You may transfer the software to another computer that belongs to you. … You may not transfer the software to share licenses between computers.

But what if I have a bunch of PCs with a mix of XP/Vista/Windows 7? Can I purchase either the Windows 8 Pro Upgrade $40 (download only) or $70 (DVD) version (both of which come without a key) only once and use it to upgrade all the PCs? Since I'm not sharing the license and each PC has its own valid genuine license, it should be allowed, right, or is it illegal? Even if they want people to shell out $40/$70 for each PC, how would they enforce the use of the installer/media on only one PC each?

EDIT: I have been given to believe by a source that the installer will only check for the previous OS' key, which is what is confusing me (I have never purchased an upgrade version before this, only full retail or pre-installed versions). Is this true or will I need to enter two keys to make the upgrade work, one for the previous version and then one for Windows 8? If the latter is the case, then the issue is solved since obviously the same Windows 8 key will not be valid for multiple PCs.