Mike-
My company recently changed one theatre in my area to self service soda. They order from the concession attendants, and are given an empty cup that they take to the fountain to fill. They are saying they are able to speed up transaction time, thus being able to serve more patrons quickly, avoiding the walk-aways and people who just avoid the stand altogether because of lines.
We offer a free refill on our large size only, and they have said that so far they haven't seen too many people trying to fill up the smaller sizes for free. Just be sure that you have a clear cup for those people who ask for water, so you can be sure they are not filling that with soda. I can't see anyone bringing their own cup from home...