I second mtn_climbr001's response. On a phone call with the Forest Service last year I was told that they want to promote the use of the main trail as a relatively safe walk-off for people climbing Whitney via the MR or any of the technical climbing routes. Their thinking is that an exhausted climber should not be forced to down climb the MR because he ro she lacks a permit. The risks to the climber and potential rescuers are too great should there be an accident during the down climb. (Climbing statistics suggest 85% of climbing accidents occur on the descent.) We are planning to do some climbing in the North Fork canyon this summer and I have spoken with the FS about permits recently. You do not need a permit for a one day trip in or through the canyon. You need an overnight permit if you plan to camp. The overnight permits, which are not part of the Whitney lottery, are available 6 months prior to departure and go very quickly.