I've consulted in the hospitality industry and also trained employees in all types of jobs. I figure, I tip the bell people with my luggage and the restaurant people, too. When we do a taxi ride we tip the taxi person, so I just extend it to other people.
Now, I only tip for good service. If what I receive is medicore then I don't tip.
As far as housekeeping goes, I think that the job they do is as difficult as that of a waitress (I was a waitress for many years and did "shadow" housekeepers in a hotel). I tip nicely the first night I am there to "set the tone" and it almost always works well. They then go above and beyond to do a nice job and get me anything I ask for, within reason.
We, also, tip whenever we are at a hotel, not just Disney. Sometimes if we are only staying one night, I forget to do it. But, in general we typically do tip housekeeping.