This Applescript could fix a broken OS X Software Update in the case of an error of this kind: "Make sure you have permissions to write to /tmp/501/TemporaryItems/com.apple.SoftwareUpdate".
This script copies the "tar" file needed by Software Update to the right location. The "tar" file not being there is the cause of the problem, which is thus fixed by this script. This script also repairs permissions in case that could be part of the problem.
Version 1.1: Now includes the tar file. Some OSX installations seemed to miss the file on the assumed location. This release should fix this.
Mac OS X 10.1 or later