It never occurred to me that people tip housekeeping... We're from UK where we only tip in restaurants (and even that didn't used to happen a couple of decades ago - tipping is pretty rare in Europe still, and our French friends have to remember to tip in restaurants when they come here) - when we come to US I remember to tip bell services and ME driver. Who else are we supposed to tip?! I'd really like to know!
For people who tip at end of stay: do you assume that it is the same person who has done the housekeeping for T&T during your stay? Couldn't it be different people?
Also, this is terribly basic I'm sorry, but where do you leave the tip - if it's during your stay how do they know it's for them and not just your money on the table? Do you put a note on it?