I'm running Debian Sid here, with GNOME 2.8 from experimental. When I start GNOME (through KDM/GDM/XDM/Whatever), I get the splashscreen, then a gnome-terminal. I eventually figured out I had to start metacity manually from this terminal. If I do that, GNOME eventually starts up, 3 to 4 minutes later, and I can kill the terminal - metacity stays running.

Is there any way around this?

In true Gnome way there is a wrapper inside a wrapper inside a wrapper to run the window manager and any of these can be broken..

First thing you should try is to run the Gnome configuration editor and check under
/desktop/gnome/applications/window_manager/default entry to see what window manager it is set to run by default.

Default is blank, current is /usr/bin/gnome-terminal.wrapper

Which explains why it launches Terminal at startup then...

However, gconf-editor doesn't want to let me set a default or change the current one as a normal user, and as root they're both blank.

By "doesn't let me", I mean it acts as if they're saved but if I reopen gconf-editor, they're not.

Take a look in /etc/gconf/2/paths file

This lists the places gconf configurations are loaded from. From these you can check/change default configurations. You ought to have some "readwrite" locations there where lamers could write their settings.

Tried changing the settings manually in $HOME./gconf/desktop/gnome/applications/window_manager - this time a change to default holds but current changes back to gnome-terminal.wrapper as soon a gconfd-2 restarts

On running gnome-session from a safemode xterm, I get the error "WARNING: No window manager could be found". This is an error from the gnome-wm script I think. On checking that, the script variable DEFWM wasn't set. Setting this by hand to /usr/bin/metacity had the same result - nothing, except the addition of a warning from a crashing gconfd-2...

I can hear XFCE4 calling me in the distance at this stage... I want a GTK2 DE, but GNOME seems to be a total *****

I think the word "experimental" says it all..

Check from which configuration file that terminal wrapper is taken from and then manually edit it to be the metacity exacutable.