I guess it could be installed on either. From the Upgrade FAQ.
Do I have to install the Windows 8 upgrade on the eligible PC, or can I install it on another PC?
The upgrade offer is tied to a Windows 7...
Maybe a dumb question: I have a Win7 desktop and may buy a Win7 laptop. With the latter purchase, I could get the Win8 upgrade for $14.99. Would it have to be installed on the laptop, or could I run it on the desktop (which I'd rather do)? Thanks.