Thanks, but that way around I know about.
From the “Blender Experimental 2012” key-map I have found the following code that makes a menu with the available modes.
""" A menu for switching between object modes.
bl_idname = "OBJECT_MT_mode_switch_menu"
bl_label = "Switch Mode"
def draw(self, context):
layout = self.layout
# Temporary work around: Blender does not properly limit the mode switch menu
# items until the first mode switch (e.g. mesh objects will show pose mode as
# an option).
# TODO: file a bug report for this behavior.
So this is almost what I want, but I don´t want the menu.