Problem: Found out today when setting up first Windows 8.1 Pro computer that it would not show up in WSUS 3.0 Sp2 (3.2.7600.251) running on Server 2008 R2. The new computer did show that updates were managed by WSUS (system administrator) however it gave an error when you checked for updates.
Resolution: You need to install KB2734608 on the WSUS server, reboot, synchronize and then follow the instructions on it to clear the Windows 8 / Server 2012 computers SoftwareDistribution folder.
After you update your WSUS server the version changes to 3.2.7600.256
See here for the download link and instructions: http://support.microsoft.com/kb/2734608