OS X: hidden frames / no keyboard commands available

[David Reitter]

When the last frame is hidden, Emacs (Carbon Emacs) does not seem to 
accept keyboard input any more, even though the menu bar displays a 
range of commands as being available, depending on the mode of the last 
shown buffer.
For example C-x C-c should allow me to exit Emacs in this situation.

This happens on a recent CVS build (2005-04-15), on OS X (Carbon).