Ok got it figured out, OnSceneGUI() in a editor script can be used to do things when the specific object is selected in the scene.
answered Mar 25 '12 at 06:57 PM
Inside this class you have the target variable which holds the reference to the classinstance you've selected.
Note that Update isn't called periodically. It is called when something changed (you moved the camera, an object). If you need a periodically event There's also a delegate in EditorApplication
answered Mar 25 '12 at 12:16 PM