I don't know whether the problem I had was related, but I walked away from the install on my Early 2008 Mac Pro, and when I came back, the screen was black. I ssh'd in from another machine to reboot it, and watched as it started booting, and then, at the moment when it normally changes screen the resolution, the screen went black again. Somehow, the update left my display settings in some mode my monitor couldn't handle.
I rebooted again, in safe mode (hold shift on startup); it booted fine (at 800x600), and I logged in to set my display resolution back to normal, and rebooted again; it's been fine since.
If you have another machine and ssh enabled, log in that way to shut it down. Or, if you have auto-login enabled (or can log in blindly), press Command-Space to bring up Spotlight, type Terminal, and press return. Then type sudo shutdown -r now and press return, then type your password and press return. Hold shift while it reboots in safe mode.
