Here's a tip I found on the Apple discussion boards to get Software Update working again.
It seems that Software Update will stop working for no apparant reason, and just deleting the plists and cache stuff doesn't help. But here's a workaround that does:
- Delete /Library/Preferences/com.apple.SoftwareUpdate.plist.
- Switch to a different network interface -- Ethernet instead of Airport, for instance.
- Run Software Update.
[robg adds: I did some digging on the Apple discussions, and I think this thread is the one the author mentioned, though there are others with similar advice.]

