Worked through a problem recently where my client PC's would not connect to WSUS 3.0 SP1. Here's the steps I followed to correct the problem.
1. Delete the PC from the WSUS console (Client PC's - The PC will likely have a red X)
2. At the client PC - Select Start - Run - type CMD
3. At the command prompt type net stop wuauserv
4. Navigate to the Windows Directory
5. Delete the 'Software Distribution' Folder
6. Run Microsoft Update from the Start menu (This will ensure the update software is up to date)
7. Restart the PC
8. Select Start - Run - type in the run box http://WSUSServerName/iuident.cab (wsus server name is the name of your server)
9. You should be prompted to download or open iuident.cab. This confirms network connectivity.
10. Select Start - Run
11. type in the run dialog box wuauclt.exe /resetauthorization /detectnow
12. Wait 10 minuites +/-. The PC should connect to WSUS and begin downloading updates.