How to switch the window manager

[Juanma Barranquero]

Warning: Newbie question ahead :)

I'm using Ubuntu, and I'm reasonably happy with Gnome but metacity is
really a bit short on features, so I'm giving a try to Enlightenment
(DR16, the just-a-window-manager version, not the shinny but yet
unfinished DR17, total-windowing-environment one).

Well, I've had no trouble getting it with

  sudo apt-get install enlightenment

but, what do I do now to convince GDM that I want to use Enlightenment
as default window manager?

Thanks,
           /L/e/k/t/u