Reasonably crash-proof smartphone mounts already exist; they just cost a bit more. I use http://www.amazon.com/BioLogic-Bike-Mount-iPhone-4/dp/B004G7... for mountain biking, and I've broken a few mounts (I haven't found a good spot to put it that's visible but not in the path of my legs when bailing off the bike), ridden in the rain and fished it out of a stream without the phone itself suffering any damage.