I think I might have found a bug in Snow Leopard. In Mac OS X, you can set it to require a password after an amount of time or immediately after the screen saver starts or your Mac goes to sleep. This morning I shut my MacBook’s lid and then unplugged my iPod, causing it to wake-up briefly before going back to sleep. I just woke up my Mac to find the password dialog with my desktop visible. Normally, you’d see a black background. To my surprise I could interact with my Mac normally while the password dialog is still open. Clicking Cancel just puts it back to sleep. Waking it up again brings me back to the desktop with password dialog.