I usually tip $4 to $5 daily depending on the hotel/mess. My friend's Mom is a maid and always tells a story about how a famous basketball player "stiffed" her when he stayed there. My husband also works at a hotel/casino and knows some guest room attendants who have mentioned they were stiffed as well. Based on that, I would say that some expect tips or why would they say they were stiffed. Most people do tip, but there are a lot that do not. They are not tipped as often as a waiter but are tipped as often as a pizza delivery person. There are a lot of people who do not tip them either thinking it is included with the delivery charge or just not wanting to.
For a maid that cleans 12 rooms a day in Las Vegas (anyway), she will usually get a tip from 6 or more of the rooms. Their salaries are not as low as waiters, because they have way less "covers" per day and more people leave without tipping a maid than without tipping a waiter (although that happens from time to time as well). Housekeepers are paid less than front desk agents/receptionists in most markets despite a previous poster's comment.
If I forgot a tip, I would feel bad too. I would not mail it in and just try to remember next time. Doubling up is not necessary, but a daily tip is better than one large tip at the end. Someone could be getting "stiffed" while someone else will be getting a very large tip if you have more than one mousekeeper throughout your stay.
Tipping is a touchy subject, since it is all optional. If you leave and feel bad about not tipping, tip next time. On the other hand, if you are opposed to tipping, you should not. Be prepared for people you tell to think that you are wrong but then again, you think they are wrong too!