I tip my housekeepers depending on how good of a job they do - first day, probably not getting a tip. Second day, if you did a nice job, big tip. Some hotels in WI are so ##@#@ lazy about their housekeeping - I stayed at Devil's Head for a College Democrats convention last year, and our towels had MOLD growing on them.
Another horror story - stayed at the Don Q in Dodgeville after SigEp banquet and there was
crayon everywhere in the bathroom. It only took like 15 seconds for me to clean up, but damned if that housekeeping staff deserves my money for doing such a crappy job.
I think most people have no idea that it is customary to tip housekeeping - it's not common knowledge.