@Zorg and Virion - dunno, I was just basing my comment on this article which says older hardware is not supported (but the installer can be hacked). Unless the article is entirely incorrect, it does appear that the OS X installer (as it comes "out of the box") will prevent installation on older Apple kit against the wishes of the owner ("Apple knows best") so I stand by my comment, even if some of you have had success.
Good luck getting Mac OS X 10.4 (or later) to run on any Apple G3 or G4 powered kit, or even late model G5s.