Thanks for all the help!
Changing the permissions for ext2fs.kext did indeed get it working.
Sadly replacing my AppleACPIPlatform.kext and AppleSMBIOS.kext didn't work - in fact, they stopped my PS/2 keyboard and mouse from working! Fortunately I had a USB mouse and keyboard lying around, so I copied back my previous kext's. So now I'm back to square one. The shutdown irregularities are not that big a deal, but when I start the DVD Player I get:
"There was an initialization error - A valid video device could not be found for playback. [-70017]"
Is this something to do with my 8800GT card? Do I need to edit some plists inside a kext maybe?

Thanks again whether you are able to help or not, Mike