According to people familiar with the matter, engineers for the company recently determined that Leopard installs on 800MHz PowerPC G4 systems ran "too slow." Support for those systems was subsequently pulled from the most recent pre-release copies of Leopard, which inform testers that the software "cannot be installed" on those computers.It's also rumoured that Mac OS X 10.6 may no longer offer support for PowerPC-based Mac systems.
Instead, Leopard will now require Macs with "an Intel processor or a PowerPC G4 (867 MHz or faster) or G5 processor." Other system requirements include a DVD drive, built-in FireWire, at least 512MB of RAM (additional recommended), and at least 9GB of hard disk space.
Though seemingly mild, the 67MHz increase will exclude a handful of Mac system, namely the 800MHz PowerBook G4 (Titanium), 800MHz PowerMac G4 (Quicksilver), 800MHz iMac G4, 800MHz iBook G4, and 800MHz eMac.
Source: Apple Insider