Hi! I'm have rpm based system with
libxklavier.i686 0:4.0-7.fc12
libxklavier-devel.i686 0:4.0-7.fc12
and xfce4-xkb-plugin.i686 0:0.5.3.3-1.fc12 not working (changing layout broken)

I compile  xfce4-xkb-plugin-0.5.3.3 from source and have this error:
xkb-config.c: In function 'xkb_config_initialize_xkb_options':
xkb-config.c:153: error: too few arguments to function 'xkl_config_registry_load'
xkb-config.c: In function 'xkb_config_get_xkl_registry':
xkb-config.c:550: error: too few arguments to function 'xkl_config_registry_load'

because in  <libxklavier/xkl_config_registry.h> two arguments instead one:

/**
 * xkl_config_registry_load:
 * @config: the config registry
 * @if_extras_needed: whether exotic materials (layouts, options)
 * should be loaded as well
 *
 * Loads XML configuration registry. The name is taken from X server
 * (for XKB/libxkbfile, from the root window property)
 *
 * Returns: TRUE on success
 */
        extern gboolean xkl_config_registry_load(XklConfigRegistry *
                                                 config,
                                                 gboolean
                                                 if_extras_needed);

I change string
	xkl_config_registry_load (registry);
on
	xkl_config_registry_load (registry, TRUE);

and install plugin. Now all works.

It means, that xfce4-xkb-plugin.i686 0:0.5.3.3  incompatible with libxklavier=4.0? 
Some ideas?