Depends on the location. Marriott Norcross (Georgia) is doing the "going green" promotion. I was there last week and was told that if I put the "no maid service" hang tag on my door before 2 am, they would not service the room the next day. They provided a $5 restaurant credit or 500 points as incentive. But that is the first time I've ever stayed at a Marriott that offered the option so I don't think it is very widespread. Most just have the "hang the towels up so we don't replace" policy.