I ride with my Blackberry in a Ziploc in the center jersey pocket with zero ill effects. Phone is almost 2 years old now, and while I'm not the most active cyclist, I have put about 2k miles a year on the bike with the Blackberry. Worst problem was when I crashed at ~18mph and put a small ding in the case (the road rash was a much bigger problem).
If you are looking for a VERY "bomb-proof" case, check out OtterBox brand cases. I work in construction, and I've seen an iPhone dropped off a 3rd floor roof, bounce across the concrete sidewalk, and land in the gutter with running water. Took a minute or two to retrieve the phone, and it was still in perfect working order. Those OtterBox cases are dang near indestructible.