When I did this, Software Update generated an error message saying it could not write to /Library/Packages. So I checked that directory, and changed its ownership, and now things work normally again. It turns out another Admin user's account on the system had been set for ownership of the directory in question, perhaps from other accounts using Software Update? I also noticed that the directory owner had write privlages, so perhaps that should be changed as well.
Hope this helps someone in a pinch.

