Hallo
EDIT error: ID is missing …
The Api doc is available see http://www.blender.org/documentation/blender_python_api_2_56_1/bpy.types.PropertyGroup.html
it is used in the very nice ruler addon
My own compilation does not have bpy.types.IDPropertyGroup available,
though IDPropertyGroup is in the source rna_ID.c.
So it is probably only a missing ‘switch’ in user-config.py?
Who knows and can tell me (us) ?