I did a security update (Security Update 2005-009) running Tiger on my powerbook. Restarted and nothing but kernel panics. I called Apple and they stated there have been no other incidents. Okay, fine. I'll do it myself. Can't figure out the problem so I did all the things you are supposed to do. Still nothing. So I boot in Safe Mode, go to my user folder then preferences and delete all preferences that had the date for that particular day. Reboot, no more kernel panic. Of course, now I have to reset my preferences for mail, network, etc. etc. Go to work. On my G5, I held off a few days. Was a little afraid to do a security update. After speaking with Apple again, they confirm, this was an isolated incident. Update, Reboot, kernel panic. Okay, it's not only me. Call Apple, they run me through the basic troubleshooting and I tell them those aren't the issues. I told them I would try a few things and get back to them. Well, I booted up in Safe Mode (hold the shift key upon boot until you have the thinking circle). Then I connected to the internet (was surprised I could in safe mode) and did a google search. Low and behold, others are having this problem. Not widespread, but there are others. I found out it was the CocktailBroadbandOptimization folder in the startup items (hard disk>Library>StartUpItems>CocktailBroadbandOptimization). I deleted the folder and walla, no more kernel panics. On my laptop, I didn't have this folder, but I did have CocktailDialUpOptimization. So for those leary about updating because of this kernel panic, simply go to this location and see if there are any Cocktail folders in the startup. If so, trash them, then update. All will be well.