I think I found the culprit. It is something about/in Internet Explorer 8. When IE 8 is open, the desktop goes unresponsive. Close IE via the Task Manager, and voila! I have desktop access again.
I'm also having this issue, on a 32-bit version of Windows 7. It started a few weeks ago, and at first I thought it was Stardock Fences, so I uninstalled and rebooted... nothing. I thought it might also be some desktop widgets and adobe air programs I have running, turned off, rebooted...