To tip or not to tip?

I recently read an article about eco-friendly hotels in which the author (the lovely Holly of Nothing But Bonfires) referenced, among other things, those signs in hotels that urge you to hang up your towels if you'd like to use them another day. The article was interesting, but what caught my attention was a comment at the bottom of the article. A reader mentioned that even when he hangs up the towels, the maid service whisks them away and replaces them with new ones, prompting him to want to lower his tip. His tip? Wait, his tip? I confess that I've been staying in hotels lo these many years and have never left a tip for the maid. Twice I've stayed in a B&B where it was (politely) suggested, but never in a hotel. What do you think? Is it bad form not to tip the maid service at a hotel?