A while back we posted an article saying that anything under an 800 MHz G4 wouldn't be supported by Leopard. That still appears to be true, but it seems that Apple has now excluded that 800 MHz G4 too, in the latest Leopard beta release. According to AppleInsider, Apple felt Leopard was simply too slow on an 800 MHz G4.
I have a 1.07 GHz G4 iBook at my disposal, and I'm still hoping that I will be able to transfer a Leopard install from that iBook over to my Cube. Although the Cube has a 1.7 GHz G4, Leopard likely will not install on it because it it only shipped with a 450-500 MHz G4. Readers have stated that similarly upgraded G4 Macs are listed as "unsupported" with beta versions of Leopard.