Since our 2nd or 3rd trip, we have always left $5 (cash) in an envelope that I prepared before leaving home using my greeting card progam and Disney clipart I downloaded from the web. The envelope is usually placed on the dresser, in front of the extra glasses and ice bucket so it cannot be overlooked. Reaction from Mousekeeping has ranged from ambivelant to totally appreciative.
During Oct last year we were at the Contemporary during Hurricaine Wilma. The day they closed the park, they 'suspended' housekeeping, but left a message on the phone that if we needed anything, just call. I called and got fresh towel (within minutes, actually), so I gave the girl the full tip. I have never seen anyone so grateful for such a small gesture. It's for that reason we will continue to leave 'Thank you, Mousekeeping!' envelopes each night of our vacation.