If you inform someone you will get back to them.. do so.  If you make a commitment, keep it. If you cannot, make sure you inform the person with enough of a lead time so that they can make alternate plans.  Otherwise what you demonstrate is a lack of professionalism, a lack of courtesy, and a lack of respect for the time of the other person.  Not to mention the fact that it will tend to royally torque the person off. This is especially not a good idea when that person is one of your best proponents and has a very public voice.