In over 15 trips to WDW I have had only one time that housekeeping didn't pick up their 'tip'....still not sure why. Anyway..I now use the paper from the notepad on the table. I write 'thanks' on it, with some impromptu Mickey heads, then fold it in half and put the tip in that. I leave it on the vanity counter.
It is true that housekeeping is not a tipped position at WDW. But, the majority of housekeepers seem to have no problem accepting the offered tips, and most of the guest to tip on a regular basis.
Now, there have been a few instances where I have not tipped. When I get back to my room at 4:00 and the beds are unmade and the towels are still in the tub, well, no tipping that day. I just remove the tip I had left that morning.