[openbox] How to use openbox on OS X?

[seanh]

Does anyone use openbox on OS X? I'm using OS X Snow Leopard, its broken
implementation of Cmd-Tab and "Spaces" is driving me nuts. I managed to install
openbox from MacPorts, but when I run openbox --replace it says:

Openbox-Message: A window manager is already running on screen 0

Is it possible to have a standard OS X session but with openbox as the window
manager, like you can with KDE or Gnome? Or is it possible to run a pure
openbox session on OS X and still have OS X stuff mostly work? I feel like I'm
asking for trouble here.