Doomsday captures all keypresses
What I mean is that Doomsday stops other programs from receiving keypresses. So it's impossible to change the system volume via my media keys (kmix), or control an external media player (various), or use any of my window manager bindings (ctwm). Happens while windowed too.
Ctrl+Alt+Backspace still works as does switching terminals via Ctrl+Alt+F#.
I'm suspecting this is a platform-specific issue as it doesn't happen on Windows.
I'm not sure how this should be handled, although other fullscreen games aren't affected in this way.