I've seen a similar issue with a system connected to an Active Directory domain. Does your system happen to be connected to AD? (If you don't know, or you're using it at home, then you're most likely not).
OK I reinstalled SL on the iMac and then checked for updates... got some but not the update from 10.6 to 10.6.2 even tried 10.6.1 and no joy there either. I guess I will wait for 10.6.3.