Software Update not updating - scripts run, but nothing happens

This is a weird bug that accompanied my Leopard update. Just now, when the 10.5.1 update came up in the Software Update and I clicked to go ahead with the system update, a dialog box asking me to reboot immediately came up. No download time or anything. I clicked to reboot, thinking that the download would occur afterwards.

But, immediately, the system went into the blue shut down screen while running the installation script, writing the files, etc. The system then rebooted and when I went to "About This Mac" it indicated that I had OS X 10.5.1 installed. But, when I checked the System directory, I found that none of the system files had actually been updated.

I proceeded to download the 10.5.1 update file from the Apple site, and everything at that point installed. That 109 MB file should have taken 6 minutes to download, but nothing with the Software Update indicated that it was downloading any files.

This is not the first time that this bug has cropped. It also occurred right after installing Leopard. Software Update showed that 3 updates needed to be installed (Keychain, Apple Remote Desktop, and Garage Band updates). I clicked to proceed with the updates, and the same thing happened. The reboot dialog immediately came up without any downloads occurring.

After rebooting, I checked Garage Band and saw that the update had not been installed. But, upon running Software Update again, these 3 updates did not pop up again. As with the 10.5.1 update, I had to download the updates from Apple's website.

Has anyone else run into this? And does anyone know what might be the cause?