If I was using my phone for Nav I would get something like this:

http://www.amazon.com/RAM-Mounting-S.../dp/B001RTOSRY

RAM makes the handlebar brackets for bikes and they make a motorcycle mount as well.

The above case would keep the phone dry in the rain as well. Personally, I have a garmin 800 i use for nav and bike stats and keep the iphone in a sandwich baggy in my jersey pocket.