OK, it turns out that Ctrl+Alt+Down is actually a standard key for flipping the screen on computers with at least some Intel video adaptors, so that's a really bad key combination to use! (Just had a user call and complain it had happened.)
You can disable that (at least on Windows 7) with the right-click menu option from your Desktop Graphics Options -> Hot Keys -> Disable. And you can undo its effect with Ctrl+Alt-Up.
So, I'm going back to just Alt-Down. It seems to me that's much less likely to have been mapped to a system hotkey.