Hmm. I have almost always found 2 free bottles of water avaialable in my hilton rooms (hhonors floor, etc). At the waldorf they tell you to just get them out of the minibar. I've never received a voucher or anything to replace them with, but the desk people cheerfully remove the charge (which is still an inconvenience since then I can't do express checkout).
Brady
(hilton diamond, stayed at many many many different hiltons last year)