This summer I picked up two Nite Ize handlebands. Although not waterproof, I have found them to be absolutely awesome. Very easy to use out of the box, secure and cheap. I would not hesitate to recommend it to anyone looking for a smart phone holder!
Here is the one I picked up: Robot Check
I have been riding these on both my commuter and road bike since May and I am very happy with them both. Highly recommeded.
Robot Check