Ever since I upgraded to Snow Leopard 10.6.1, my MacBook Pro startup and shutdown was much slower than before.
I searched for a quite a while on forums to find the solution, but nobody seemed to know a solution. I finally figured out what it was. It seems that somehow the ownership of my startup volume's root directory (/) wasn't assigned to the root account anymore, but to my account instead. This made the kernel prelinking fail since it requires that root is the owner.
This is how you solve this after launching the Terminal app:
sudo chown root:admin /
sudo kextcache -system-prelinked-kernel
sudo kextcache -system-caches
You'll have to enter your password after the first command.
Hope this helps someone, since it was really frustrating to have these slower startup/shutdown times for me.