About the technical and legal stuff:
1.) The upgrade disks (29$ for Leopard owners, 9.95$ for recent-buyers) are intended for _one_ machine only. Technically, they'll probably install on any intel Mac with Leopard installed. It's not difficult to circumvent any limitation, technically, but it's legally wrong to "give" another computer SL for which a license hasn't been bought.
2.) The "family pack" disks are for up to 5 computers in the same household. And no, that does not include that nice girl's computer living in the apartment next door or a friend's computer that just happens to be in the same household on a sunny Sunday afternoon. It's meant, as the name says, for families primarily, and its use is extended to "households" that are not, strictly speaking, families.
As a computer sales person I find it hilarious (I'm not saying you're thinking like this, btw., it's just an anecdote), when for example the father of a student comes to our store to buy a student edition of Adobe Creative Suite 4 Design Premium. It's obvious that _he_ wants to use the software as soon as he's asking about whether he (yes he) would be able to install the software on more than one computer in his (yes his) household. To people like that I want to scream: Pirate the f***ing piece of software already! Why pay Adobe a couple of hundred dollars instead of a couple of thousand dollars when you can simply download a cracked version of the same thing? It's in *no way* "more legal" to abuse his daughter's student card than to directly steal. In a strange way, pirating seems *MUCH* more honest to me, personally, than abusing a student discount in this way.
Similarly, I find it hilarious how I recently came across a few kids (classmates, I guess) who decided to buy the Mac Box Set Family pack, so they could install it on all their computers. I know it's wrong, but I sold them the single-user license instead after it became clear that they planned to neglect the terms of the license, anyway.
End of rant. Sorry to hijack the thread like this.