How to fix: “10.4.11 Broke my Mac!”
Users experienced problems such as Blue Screens when the login screen was expected, a screen that switched between a black and a blue one, or simply the mac whirly-thingy is sticking. A lot of people have ended up simply reinstalling the system.
A bit of Googling brought me to this post about 10.4.9 doing the same thing. The author claims it is because the system got interrupted while it was pre-binding after the install was done. I encountered the problem on several machines that had been forced to run software update via Apple Remote Desktop and all upgraded to 10.4.11. Nobody was actually using the machines, but it’s possible they were running processes while prebinding was happening.
So from terminal on each affected machine I ran: sudo /usr/bin/update_prebinding -root / -force -debug
And now each machine boots happily!