There have been a few prominent issues following the update to OS X 10.6.8, though a couple of them have to do with third-party application support. One that has caused a number of users to have kernel panics is the tool Hands Off, which is a personal security program. Apparently the kernel extension for version 1.3 of the program is the culprit, and should be removed.
To do this, boot the system to Safe Mode and then locate the file "handsOff.kext" in the /Suystem/Library/Extensions/ folder. Move it out or delete it, and then restart the computer. If the system will not boot to Safe Mode, then boot it to Target Disk Mode (hold the T key at start-up) and attach it to another Mac with a FireWire cable. Then use the second Mac to navigate the filesystem and remove the extension.
If you have not yet updated to OS X 10.6.8 and use Hands Off, the company has a new version (Hands Off 1.3.1) of the software that corrects the problem, so be sure to update before installing. After you have corrected the problem you should be able to install the latest update to prevent it from happening.