I know there is a login settings screen. I have it set to "Openbox Session". And every time I go back to the login screen, the default window manager is Gnome. Still.
I'm still using GDM.
The other machine I'm currently configuring -- lucid again, but with no Openbox installed -- does not have this behaviour. (Although I'm not saying that it's installing Openbox that caused it.)
If you don't have an answer, but it's happened to you, I'd still love to hear from you. Thanks.