I have diagnosed this problem on my Ibook and my new firewire powerbook....crashing finder on shutdown and bootup.
It looks to be an AppleTalk and/or Sharing problem....try doing this:
Go into your AppleTalk control Panel:
Set your USER MODE to Advanced.
Click on Options.
Set AppleTalk to INACTIVE (from within this menu)
Now try shutting down and restarting your computer....and see if it crashes with this off.
(Ive also had issues with Energy Saver - try disabling it totally - by setting all timers to NEVER enable energy saver....Im an ex Apple employee - and Energy Saver and Appletalk have been historically buggy products....I cant wait till they are out.)
AppleTalk is a vital function for many people....try just turning it on when you need it - then turing it back off.
Ive found that Timbuktu replaces most of my day to day Appletalk functionality....and does a much better job. Ive installed it on all my machines.