I'm also seeing this problem within my organization, especially on laptops with mobile accounts. Another (possibly related) issue has been that I've seen the kcm and opendirectoryd processes near or over 100% cpu utilization each sometimes.
We use AD for user authentication, and Open Directory running on a Mountain Lion Server for client management. AD is the first directory in the search policy.
When the clients hang I can use screen sharing to access them remotely, so the system isn't hard locked, it's just the lock screen.
I'm in the process of digging through system logs to try and find any clues, but in some of these instances it doesn't even look like an attempt to unlock the computer is being made based on what I've seen so far.
If I can get a really good log file sample, I'll post them up here.