Problem: Note that Alt-space as far as I know this particular keybinding is not configurable with metacity (I could be wrong though), so this bug is not a special case of the "should use metacity keybindings".
I don't think there's UI for changing it, but there is the /apps/metacity/window_keybindings/activate_window_menu gconf key. compiz has /apps/compiz/general/allscreens/options/window_menu_key which defaults to <Alt>space, which works for me here. Also I just changed it to <Control><Alt>space so it doesn't interfer with emacs' keybindings - that also works.