Thank you
ʞəʇɹɐq!
I knew the issue was somehow related to QuartzCore because Applejack reported repairing the symlinks and that it otherwise found no other problems.
When I booted from backup I checked /System/Library/Frameworks/QuartzCore.framework/Versions/A/ and QuartzCore was gone.
Following your advice I extracted it from the Safari 4 installer using Pacifist, replaced the missing file and voilà - reboot was successful!
I chose to restart and run Applejack again instead of using the terminal commands just to make sure
everything else was still functional.
Applejack:
http://applejack.sourceforge.net/#top
(I highly recommend installing this as it has rescued me on numerous occasions when everything else has failed.)