With Tiger there was no copy protection on the install DVD. Although I don't know for sure I doubt that that this will change with Leopard.
What this boils down to is how seriously you take the end user licence agreement. If you're the kind of person who doesn't mind copying software then you probably won't think twice about installing a single user copy on lots of machines. However, if you want to observe the licensing then you'll pay for the family pack. I've got two Macs and will, therefore, be buying a family pack.
It makes no difference if there IS or ISN'T copy protection on the disc. This doesn't make it any less illegal to break the EULA.
Those who are commenting that Apple is being stupid and "asking" for you to install multiple copies of the OS from a single disc: Would you expect to go into a shop and pay for one can of soft drink but walk out with an armful of them because they're on the shelf and not locked away in a cupboard?

*THINK* about what you're saying! Software isn't different to anything else and what you propose is still, when you get down to it, stealing.
Lecture over!
<Phew> I feel better now. I'm off for a lie down ;-)