I would not be offended by the presence of a tip envelope in my hotel room, I'm under no obligation to use it, just like I'm under no obligation to use the bible in the drawer, the dry cleaning slip, the Papa John's coupons, or the blaring pay per view movie channel adverts when I turn on the TV. There's a lot of stuff in a hotel room I don't need or use, one more envelope isn't going to change my opinion of the place.
That said, I usually tip housekeeping. My stays are usually longer, 8 - 10 days at a time, and I keep the 'don't disturb' sign up several days in a row, so maybe two or three times a trip, I'll have housekeeping in to clean and I leave a $5 or $10 bill if I'm satisfied with the service I'm provided. If I'm short on cash, I've never had a problem taking 90 seconds out of my day to use the ATM in the lobby and stop at the front desk to get change for a 20 on my way in or out of the hotel that day. If my bed isn't made, toilet paper not replaced, trash not removed, or any other issue I feel should have been handled by the housekeeper, I'm less likely to tip as much, or at all for that particular occurrence.
As a matter of personal preference I also tip my barber, skycaps, bell persons, concierges, taxi drivers, tour guides, and waiters/waitresses. Many people who have posted here seem to take offense at leaving a tip for housekeeping, yet they state they do tip other professions as well. What's the difference? Why should Marriott suffer your ire for not paying housekeepers 'enough' but you're okay with tipping the bellman at the same hotel? Why don't Yellow Cab, Ruth's Chris, or Olive Garden share a similar disdain?
Not sarcasm... sincere questions. Why is it okay to withhold tips from housekeeping if the service they provide should be included in the cost of the room and reimbursed to them by the employing company but you go ahead and tip your cabbie or waitress? Shouldn't the service they are providing be included in the cost of your fare or meal and shouldn't it be the responsibility of the company they work for to pay them instead of you having to tip them? Shouldn't you be withholding all tips if that's how you feel rather than picking and chosing which tipping conventions to follow?
Personally, I think some people are simply more generous than others. Some people like to share what they have, especially with those who provide a direct service for them. Some people are raised or have raised themselves to value their money foremost and don't like the idea of being put on the spot or feeling obligated to part with it. Some others may honestly be in finacial situations where they cannot afford to tip. There may indeed be more people who don't tip housekeeping than those that do, so be it... I don't look down on them or expect them to do as I do. As for me, I plan to keep on tipping.