WSUS Clients Showing Unknown in Reports Status of Computers
Sometimes, after you approved some updates you will notice many of the PC's are reporting status as UNKNOWN.
This is a known issue if you just approve an update and then check the reports. After approving update, if the clients did not perform a detection cycle, you see a status of UNKNOWN. Yes, TIME is the factor here.
You can force update detection from the client and you will see the status no more remains Unknown.
MORE INFORMATION
Quick Automatic Update Client Detection & Installation with Windows Server Updates Services:
http://msmvps.com/athif/archive/2005/06/29/56200.aspx
WSUS: Script to Force the Update Detection from Automatic Update Client for updates on WSUS Server:
http://support.microsoft.com/kb/555453