Watching various forums and newsgroups there are some common trends related to missed configuration steps which are causing some clients to not 'check-in' with the WSUS server.  There are also 2 known issues one which is fixed the other to be shortly.  Oscar Lee and Don Cottam and I pulled the following information together and posted it here to the WSUSwiki. Hope this helps.

Forget that Bobbie_Harder blog......THIS is the REAL WSUS product team blog where we will be posting product information, tips and tricks, best practcies ,and other tidbits we find and hear not only from us, but from across the community!   We welcome your comments and hope we can help you get the most out of WSUS and help us learn how you use it!  We'd like to learn what you like, what you don't, and what you'd like to see in the future!! Team members who will be posting to this blog will be Developers, Test Engineers, and Program Managers such as myself.  We're all  really excited about this product which just released June 6th, and we're hoping our cross team perspective will prove to be useful and relevant to assist you in your WSUS deployments and day to day operations.   We are all about keeping your environments secure and up to date in the most efficient and easiest to manage way!! Check back with us often!!