⌘Q (Command-Q / Cmd-Q) disappearing

This is an odd, though fairly minor, problem, and I haven't seen anywhere in the forums that anyone is experiencing anything similar:

Lately, when I've been using an app in Snow Leopard, and I'm ready to quit, I press ⌘Q on my keyboard (aka Cmd-Q, Command-Q or Apple-Q) and nothing happens. When I go to the File menu, the "Quit <appname>" option is there, but the ⌘Q keyboard shortcut is missing!

All the other keyboard shortcuts for the menu appear (i.e., Open - ⌘O, Close Window - ⌘W, etc.). Just the Quit item is missing the ⌘Q shortcut.

I can still quit by selecting the Quit item from the menu.

What's strange is, once I quit the app, if I open it again, and immediately go to the File menu, the Quit keyboard shortcut is displayed. If I work a little while, when I go back to the File menu, the Quit command shortcut is gone. Once, I even opened the menu, saw the ⌘Q shortcut and it disappeared while I watched!

This has happened with random applications, both Apple and third-party. I haven't been keeping track, but I believe I've seen it in Safari, Text Edit, Expression Media, Quicken and several Adobe Apps. I've restarted the Mac, and the problem does not go away. It doesn't seem to matter which app is open, nor how many apps I have open. It's random and not reliably repeatable, though it's been happening for at least a couple of weeks now. (It may have only started with 10.6.1, but I cannot say with certainty.)

Is anyone else seeing this? Does anyone have a possible diagnosis/cure?

Thanks.