Depends on your needs. I use a Tigra case/mount for my iPhone 4 for regular biking (for exercise). Semi-water resistant (for light rains). I'd put a plastic bag over it if you hit heavier rain though.
I use it with a biking GPS app to track my route and time, etc. Not perfect, but easily viewable. I don't recommend trying to change maps/podcasts while actually moving though. Great way to crash.
This may not be the best solution for you though. Main issue for regular commuting is that you have to take the iPhone out of any current case you have in order to fit it in the Tigra mount. Means a lot of switching around of cases every day. Also, it hides the photosensitive sensor, so you need to turn off auto-brightness and set brightness up high in order to see the screen (bad for battery life).
However, the bike mounts that are more universal also leave your phone open to rain and damage (and possibly popping off the mount and hitting the ground).
If I was doing this more for commuting, I'd probably look more for a forearm strap/band.