My wife and several friends bought one of these - http://www.amazon.com/BikeMount-iPho...9340509&sr=8-2

It seems very secure; I watched a video where it was attached to a bike and banged around a bit and still did not come off.

If you are wanting something that can track cadence and HR then you may be interested in something like this http://www.wahoofitness.com/

It is the first case with the technology to give your iPhone ANT+ technology which will work with a cadence sensor and HR monitor. I would recommend the Garmin GSC 10 for Cadence.

Good luck!