and wanted to say THANKS! Housekeeping is one of the hardest working, grossest, high pressure jobs to be found in a hotel.
To say that they are not considered "tipped" positions would be correct, no Housekeeping Department I have ever seen was, as they are paid usually slightly higher than minimum wage. But that does not mean that they are NOT tipped regularly, or that it is not neccessary or polite to do so.
$1 per person per night is the usual. If you do not have a Mousekeeping Envelope then laying the dollar bills on the pillow is correct. Housekeepers are VERY concious of not taking money out of the room if there are not SURE it is for them. This is due to the high number of accusations made regarding theft from rooms. While I am sure it does happen occasionally, I investigated 100's of cases of theft by a Housekeeper and NEVER found one to be true, and usually were items found later in an unusal location in the room (under the bed, top drawer of night stand). But they almost all have been accused of taking something from a room if they have been a Housekeeper for any length of time, which is why they will NOT take cash in the room unless they are sure it is for them. This is why either the Mousekeeping Envelope or on the pillow.
Also, since the subject came up

a few other tipping standards for my friends...
Valet $3-5 at PICK UP ONLY (no need to tip the guy you drop it off with, they all share)
Doorman not usually neccessary unless they do you a favor (get a taxi, give directions, etc.) they also share with the Bellmen
Bellmen $1 per bag with a $5 MINIMUM
Concierge $10 per half hour of time more if they pull off a miracle for you.
These are all based on SERVICE and are not mandatory. If someone is rude, sloppy, or presumptous then do not tip them, or tip less. And these are only guidelines, of what they generally considered "standard" is. The same as waitstaff 15-18% being the "standard" for a server.
And again, THANKS!