I ran the Software Updates last night (January 15, 2008: iMovie 7.1.1, Front Row 2.1.2, Quicktime 7., and iTunes 7.5. for Leopard 10.5.1). At shutdown, my iBook G4 (1.2GHz, 1.25GB RAM) stayed at a blue screen for about 45 minutes until I forced it down.
When I restarted, the Setup Assistant came up and asked me to set up my user account (!?!). Not wanting to erase my main account, I created a dummy user and proceeded. When the Setup Assistant finished up, I hit "done," and voila, ... I came right back to the Setup Assistant. Over and over again.
I'm currently cloning my drive over to an external in preparation for a clean install. Just thought you other 'Fritters might appreciate a heads-up!