That's what this article in the New York Times suggests.
What's appropriate?
http://travel.nytimes.com/2012/04/15...ares-tips.html
Q. Who and when should you tip?
A. Tip your housekeeper every day because it’s not always the same person cleaning your room. If it’s a three-star hotel, give $3 or $4; if it’s a four-star, give a little bit more; if it’s a five-star you can afford $20 or $30 a day. A doorman makes an hourly wage and it’s less than anyone else’s in the hotel. If he does his job and is making my life easier, I always tip him.