Ctrl+Alt+pretty much everything else works. Ctrl+Space works. Alt+Space works. But nothing seems to see Ctrl+Alt+Space.
If I go into System Settings->Keyboard->Shortcuts and try to assign C-A-S to a shortcut, hitting the chord doesn't seem to do anything (that is, the shortcut isn't assigned), but hitting Alt+Space does.
Emacs also doesn't seem to ever receive the key, which is the real problem since Ctrl-Alt-Space is kind of important for certain programming languages. Plan B, in Emacs at least, is the key sequence ESC Ctrl+Space.
I just noticed: Ctrl+Super+Space also doesn't seem to be passed to applications, again, including the keyboard shortcuts app in the System Settings.
Best Answer
Unity maps Ctrl+Alt+S to Windows > Toggle Shaded State, whatever that is. If you disable this shortcut, then its passed to whatever application is in that window, including
emacs. It is quite confusing though.