The Mac OS 9.1 CD that ships as the Classic environment with OS X will not install on the Titanium PowerBook G4.
Macworld Sweden reports that on trying to install the CD, the following error message is given: "This program cannot run on your computer. See the documentation for more information."
Macworld Sweden tried to install the OS from the CD on both 500MHz and 400MHz Titanium PowerBooks.
A note on Apple's Technical Information Exchange (TIL) said: "A flashing question mark appears if you try to start up an affected computer from the Mac OS 9.1 CD included with Mac OS X 10.0."
The problem occurs on PowerBook G4s, Power Macintosh G4s that shipped with the Apple Pro Speakers, and early 2001 series iMacs.
The solution, according to Apple, is to: "Use the Mac OS 9.1 CD that came with your computer. Because your computer was produced after the release of Mac OS 9.1, it requires the computer-specific version of Mac OS 9.1 that came with it."