After recently getting 3 Mac Pro's in the office I realised that we had a problem. Two of them would not shutdown, they would immediately restart. If I tried to restart them they would kernel panic. These two machines were cloned from the first one and I could work out what was different between them. I then realised that these 2 machines had the Cisco VPN client (4.9) installed.

I had to install it on the machine that was working. Low and behold it then did the same thing. I uninstalled the client when I was done and the problem went away. Hopefully Apple with 10.4.8 or Cisco will fix this soon.