I have not seen that before. How does the size of the box compare with those times when it has worked correctly?
Is it the normal box, with the copyright warning on the bottom or some other box.
Are you running OS X or OS 9 on that PB? (There was a version of Software Update for OS9)
If running OS X run the Disk Permissions, in the Disk Utility and see if it makes any difference. There is currently a Security Update for G4 Macs.
Anyone else have this problem? When i run software update i get the panel that says there is an update available for my computer but it is blank. No description, size,version or anything. I have a powerbook G4