what cant iBook G3's or iBook G4's run leapod??
The official requirements of leopard have not been made public, as far as I know.
The minimum requirements are almost certainly greater than or equal to the requirements of Tiger -- i.e. Macs without built-in Firewire will not officially be supported (although by hack, almost any Mac that can run OS X will run Tiger).
Some people have voiced the opinion that the G3 processor era will be dropped with Leopard. Really, the only thing that differentiates a G3 and G4 in code execution, for the most part, is that the G4 supports a set of vector processing operations called Altivec™. The chances of Leopard relying heavily on Altivec, however, are rather low, considering it does not exist on Intel processors.
Given that the G5s are still relatively recent, there is almost no chance they will not be supported.
So basically, the only reason Leopard would not work on a G3, or a G4 for that matter, would be if Apple made it not work on them because they felt they would perform too poorly. If that happens, I'm almost certain a hack will surface, like XPostFacto, that allows Leopard to run on G3s and G4s.
Leopard is widely expected by the geek community to run fairly well on G4, G5, Core, and Core 2 processors with adequate RAM, but the final word has to be Apple's.