Well I just got my May issue of Good Housekeeping. On page 33 Peggy Post has an entire article regarding tipping, being a good houseguest and hotel guest. She says that the modern "correct" tip is $2 per day at a moderate hotel and $3 to $5 in a deluxe hotel. She also says that tipping daily rather than when you check out ensures that the tip will go to the person who did the work. I thought that was rather interesting.
Side note: when we did our first grand gathering in June we ate dinner at a sit down restaurant in MGM. The service was beyond bad, the food was decent, but the service was just awful (and 4 in our group have lots of experience in food service). But because we were a group of 8 or more a 20% tip was automatically added on. Well this came to over $60 for the tip. Needless to say we spoke to the person in charge, and had the tip removed from the bill, we left what we felt was a more appropriate tip on the table. Out of 12 days this happened only the one time. But I think the waiter knew he was going to get that automatic 20% so he could concentrate more on the tables that he had to actually earn the tip off of. So please, if service is bad, politely let the person in charge know, if no one ever does this no one will ever know that improvements are needed.