I ran Software Update this morning, to install the recently-released updates. The installation went fine.
I tried to run Disk Utility afterwards, to repair permissions. About 1/3 of the way through the operation, Disk Utility quit with the error message you see in the attached pic.
I soon found that this problem is common to Panther users, judging by discussion threads on various forums.
suggests two possible workarounds: deleting iTunes 6.0.2, or the Chess app. I thought I'd try the latter first, not wanting to screw around with iTunes, and also because I never use Chess anyway.
After Chess was deleted, Disk Utility repaired permissions normally.
Not sure if this happens in Tiger as I'm still using Panther (for the time being).