Slate's advice columnist deals with people who claim to have never heard of leaving a tip for the maid. (There's another one on the second page.) Are they serious? I can't remember not having heard of the idea.
The big question, though, is: when to leave the tip? Slate says just once, at the end of your stay, and that's what my mother does, also. I, though, follow the practice preached by science fiction conventions, which always advise their clueless nerd attendees to "Tip housekeeping every morning, not just on the day you check out, because you may have different service staff on different days" (quote from issue #2 of the newsletter of this year's Worldcon). Slate acknowledges this fact, but says, "I once asked a maid if they prefer to get the tip daily or when the guest checks out. Since maids are not supposed to touch money, she said at the end, which could mean that the woman who cleaned your room most of the week may not end up with the tip." So, tough for her, I guess.
I'm not sure I follow this argument. If maids are not supposed to touch money, then how do they pick up the tip on the final day? With gloves they don't wear on the other days? And if they're not supposed to touch money on the previous days, why do they almost always take the tip? (Once in a rare while I'll stay somewhere they don't; the last such occasion was at a small, rustic roadside motel four years ago.) And if you're not supposed to leave a tip daily, then why do many hotels now leave a daily envelope for it? At the Mythcon hotel in Albuquerque, this envelope had a form printed on it to leave requests for the maid. On the first day, we wrote, "Extra pillow and towels, please." The maid took the tip, but didn't bring the goodies. It was an inadequate hotel in many ways, but we kept leaving tips.