Compressor 4 and Qmaster completely broke down on my MacPro last month and I could no longer use the QuickCluster function for fast MPEG or h264 encoding. The sign that something was wrong was when launching Compressor the computer asked for a system password. I followed Apple's advice and created a new boot disk with only FCPX and Compressor 4 installed but that only worked for a week before it stopped working as well. BluRay encoding was not affected. I was forced to use Adobe Media Encoder for fast DVD and YouTube encodes. It's not a bad program but I like Compressor a lot better. I sent in a complaint about the problem to Apple. This week, after installing the Compressor update, I could reset the QuickCluster and it is working again.

Thanks for sharing!
(didn't have the problem myself, but it's good that people post their solutions like that, it can only help)

Something is very wrong with QMaster and my 2009 MacPro. It makes little sense as it breaks on my regular system boot and on my emergency boot disk that has nothing of my other system disk.

I am having a similar problem. My QuickClusters are not available from the pulldown menu when I go to submit a job. My problems started after the update. Everything was working fine before.
I am also using a 2009 MacPro.

I believe it's related to the 2009 MacPro and not other models. A helpful soul over at the Apple support forums suggested that I clear the Compressor background cache and uncheck "share unmanaged services with othe computers on the network". After some fiddling that worked but opening Share Monitor seems to cause the problem again. Clearing the background cache brings back the QuickCluster but not always immediately. Frustrating.