We knew that a DVD drive, built-in Firewire, and at least a G4 were required. However, one additional requirement now is an 800 MHz G4. This basically leaves out everyone with early Power Macs, including:
G4 350-400 MHz Yikes (PCI)All G4 iBooks should be fine, but Leopard will exclude most of the G4 Titanium PowerBooks (up to 667 MHz). Fortunately for most G4 iMac and eMac owners (with DVD drives), only the very bottom end 700 MHz models are excluded. Curiously, a GPU capable of supporting Quartz Extreme is not listed as a requirement, but that may be moot as I believe all 800 MHz G4 Macs shipped with Quartz Extreme support.
G4 350-500 MHz Sawtooth (AGP)
G4 450-500 MHz Cube
G4 400-500 MHz Gigabit Ethernet
G4 467-733 MHz Digital Audio
G4 733 MHz Quicksilver
This is big disappointment for those of us with Cubes and such, but we shouldn't give up just yet. I suspect that if one has access to a PowerPC Mac which meets the requirements, one may be able to transfer a Leopard install from that to an unsupported G4 Mac. (An Intel Mac will not work, since its installs are not bootable on PowerPC Macs.) The other question is whether or not those of us with upgraded CPUs will be able to install Leopard natively. Lastly, I'm sure that XPostFacto will be updated when Leopard ships, to add support for these vintage Macs.