If the reservation did not require a credit card hold, there are no consequences for just not showing up. However, it is suggested that you cancel as soon as you know you will not be attending the meal.
If the restaurant has a credit card hold, it is a $10 per person fee ($20 at Victoria & Alberts) for no-shows and late cancellations.
At prepaid reservations (CRT and dinner shows) a no-show forfeits the full price of the meal for the entire party.
Moving this to the Reservations board. See the sticky thread on that board for a list of credit card hold restaurants (there are nineteen, and four prepaid restaurants), and cancellation policies.