Had this same problem on one of our machines runnign 10.8.2, went on for weeks, so decidedly NOT a server unavailability issue. Tried several of the suggestions here (DNS, Deleting softwareupdate.plist files, etc) to no avail. Machine is bound to OD server running 10.7, policy dictating SW Update services. Had to remove both user and machine from OD, then everything worked perfectly. Hope this helps. Now I need to look into getting our server on 10.8 so it actually works.