Yes, my N900 froze this evening - something in the web browser. Opening the lens cover generated enough of an interrupt to unfreeze.

The browser window I had open was gone when I looked at the task manager.

Interestingly, the menu that appears when you hit the power button appeared as soon as the device unfroze - as if it was queued to display, but couldn't because something was blocking.

All the above was without anything from extras-testing or extras-devel installed. But I doubt I could reproduce the scenario clearly enough to file a bug.