Similar problem here on my MDD 1.25_GHz. I started iTunes while the Updater wrote the new files, which suddenly sounded like a broken CD (the same short burst of music repeating every second), and crashed the machine.
After reboot, the startup screen didn't change for half an hour. PRAM zap didn't help either, so I reinstalled 10.3 and re-updated to 10.3.4.
I love OS X! A full crash causes only one hour to reinstall and re-update the full system! Try that on Windows...