Repeated failed Software Update since upgrade to Mountain Lion
Since upgrading to Mountain Lion, I've been unable to successfully run Software Update for Apple software. This includes iPhoto, iTunes, OS X itself, and so forth, though other App Store purchases come through fine. sudo softwareupdate -l asks for an admin password, hangs for a while, then quits without error ("No new software available."). I've attempted reinstalling Mountain Lion (complete with a clean installation, then restoring files back from Time Machine), deleting the .plist file for Software Updates, and resetting the PRAM and system clock.
Of note, my install log is filled with hundreds of repeats of the following entry, though with different dates:
Oct 9 17:14:11 ***.local Software Update[3558]: Failed Software Update - trust evaluation failed in SecTrustEvaluate with result: 3
Oct 9 17:14:11 ***.local Software Update[3558]: Can't load distribution from https://swdist.apple.com/content/downloads/24/12/041-5684/xBZPYvpkKVWptD5BpKbpbh PSDyp87s553f/041-5684.English.dist.gz: Host cert invalid or otherwise insecure download
This seems to indicate some sort of problem with my certificates. I've attempted a keychain repair, but Keychain Access finds nothing wrong. What else can I try?
MacBook Pro (15-inch Early 2008), OS X Mountain Lion (10.8.2)