This problem has nothing to do with our processor upgrade being in the machine or with the firmware update that was done beforehand.
It has everything to do with how 10.5 was installed. Doing the 10.5 install as a simple "Upgrade" rather than using some of the other options like Archive and Install or Erase and Install.
When you do a straight upgrade, the installer just overwrites the old 10.4 System folder and that can leave some incompatible kernel extensions lurking in the new 10.5 folder whose response to that is to shut down on boot if it encounters something it doesn't like. So you need to do an Archive and Install of 10.5. This will create a new System folder, migrate your various user settings but it won't migrate any outdated stuff to the new folder.
