We've had some customer requests for backend SQL cluster support in the next version of WSUS.  Currently, this is not an officially supported configuration for WSUS, but we have had a customer or 2 successfully set-up this configuration.  While satisfaction, results, and mileage may vary; we wanted to share the process these customers followed to achieve this configuration.   Check out the steps here on the WSUS wiki - if you discover anything new, or have interesting results please share your comments on the Wiki page!

http://www.wsuswiki.com/UsingSQLCluster