Kappy is correct, I know people are out looking for a quick fix and it might work or not, but with Apple's products it's sometimes best to do it according to their proceedure.
I've outlined the restore proceedure here, it covers a lot of bases and explains what is going to occur so please don't be intimidated it's a LOT easier that it appears.
Take your time and comprehend the steps involved and you'll be backup up like before in short order.
Security Update 2012-001 Bricks Rosetta programs
Restore method.
#1 Backup your personal data off the machine.
Backup files off the computer (not to TimeMachine). If you don't have a external drive, get one and connect to the USB/Firewire port and simply drag and drop copy your User folder to the external drive, it will copy all your files. It's best to have two backups of your data off the machine when trying to restore.
Disconnect all drives now to prevent any mistakes from occuring.
#2 Try Restoring from a TimeMachine or hold option bootable clone
If you have TimeMachine drive, or a hold the option key bootable clone use that to erase and restore from before the Security Update occured.
Obviously if you didn't make a recent backup before the update, your going to have to skip to Step #3.
Note: A TM restore can take a awfully long time, a clone restore takes under 2 hours in most cases.
If you don't know how to restore from TM drive
http://pondini.org/OSX/Home.html
#3 Reinstall OS X 10.6 from disk
If no recent TM or bootable clone then:
Get out your 10.6 install disk and make sure it's clean and polished (very soft cloth and a bit of rubbing alcohol, no scratches) If your disk is borked, you'll have to order a new one from Apple with your serial number.
Hold c boot off the 10.6 disk (wired keyboard, internal optical drive), use Disk Utility First Aid to >Repair Disk and Permissions (do not format or erase!!), Quit DU and simply re-install 10.6.
Note: Simply reinstalling 10.6 version from disk (without erasing the drive) only replaces 10.6.8 with 10.6.x and bundled Apple programs, won't touch your files (backup anyway) or most programs, unless they installed a kext file into OS X itself. (only a few on average do this)
Note: If Disk Uitlity can't repair the drive, then come back to seek further assistance and make sure you have a copy of your data off the machine.
#4 Update to 10.6.8 without Security Update 2012-001
Reboot and log in, update to 10.6.8 via Software Update, but EXCLUDE THE Security Update 2012-001 by checkinig the details and unchecking the blue check box.
#5 Reinstall any non-working third party programs
When you reboot, make sure to reinstall any programs that require kext files installed into OS X, you'll know, they won't work when you launch them or hang for some reason.
If for some freakish reason you get gray screen at any time when booting (possible it might occur when you reinstall older programs), hold the shift key down while booting (Safe Mode, disables kext files) and update your installed third party software so it's compatible with 10.6.8.
https://support.apple.com/kb/TS2570
Note: For most users likley Step #3 and #4 is likley all that is needed to be done.
That's it really. 🙂
Further Reading:
Learn how to have a OS X drive restore ability using TimeMachine or cloning software
Learn how to clone your entire OS X boot here
http://www.bombich.com/get_ready_for_lion.html
TimeMachine help here
http://pondini.org/OSX/Home.html
Make copies of your original Snow Leopard
http://www.walterjessen.com/make-a-bootable-backup-snow-leopard-install-disc/
Learn how to make a bootable SL USB thumb drive
http://www.maciverse.com/install-os-x-snow-leopard-from-usb-flash-drive.html
Deciding on upgrading to Lion?
Learn which programs ARE NOT compatible/updated for Lion, Rosetta programs will not run on Lion.
http://roaringapps.com/apps:table
Unverified hacks
Note: You may see a "hack" for this issue floating around, it has had mixed results as people's software is different.
You may want to try it, but it's at your own risk. It has worked fast for some and not at all for others, hopefully over time it will work for all.
The above methods I outline are Apple approved restore methods + based upon experience here and not a quick fix, but are sure to work as Apple recommends such steps themselves.
If at any time you feel this is beyond your capability, please seek local Mac/PC professional assistance before you lose your data.
In fact my steps above are very simple, you might want to have a family member or friend perfrom the proceedure in about a hour or two.
If you don't have any backup, please maintain at least a TM drive, just hook up a new blank external drive and TM will ask you to make a backup, takes some time so let it work.
If your a bit more skilled, a option bootable clone is much superior than TM drive.
Good Luck 🙂
