As Ehren mentioned, it is possible. A good example is AngryAnt's Behave, which implements a behavior tree creation user interface on top of Unity, or his Path that was recently released as GPL, and that extends the inspector for specific objects.
answered Nov 25 '09 at 10:56 PM
For the most part you can add UI elements to the editor, however you can't create a new 3D view (Scene View, Game View, PreviewRenderUtility such as those used in the model importer), which is a real bummer.
answered Feb 02 '10 at 08:52 AM