They ship different restore DVDs with different Macs now. The Restore DVD (the one with OSX on it) only includes the bits necessary for the particular computer it shipped with. I suppose you could potentially try to use a PowerBook DVD on an iBook but only because the two are so similar. However, using an iBook DVD on an old Power Mac would probably be buggy.
I think they started doing this with Tiger, but maybe Panther.
However, the retail Tiger DVD that Apple sells contains all the necessary files and packages, meaning it can be used on a much larger range of Macs (and any of the new ones). 🙂