10.5.3 hang on startup
Symptom: after installing 10.5.3 your computer will not get past the initial grey startup screen.
Verify that directory services is causing the hang by booting into verbose mode. (Apple-V) You should see repeated messages about directory services crashing.
Fix:
-boot into single user mode (Apple-S)
-mount your drive in read-write mode
mount -uw /
-boot into single user mode and remove (delete or rename) this folder:
rm -ri /Library/Preferences/DirectoryServices
When you reboot these files will be recreated with the defaults.
NOTE: this will delete any configuration you make in directory utility.