The iphone has nearly all the tech specs to be the BEST bike computer on the market, even compared to Garmin's top of the line apps. It's simply way more powerful in terms of software/hardware than even the best Garmin units.

The one thing that's severely limiting its use on bikes preferentially: BATTERY LIFE.

Most accounts of use of GPS-apps on the iphone (like runkeeper), report near completely battery drain after an hour of use when recording data points as a Garmin would. An hour is really short to begin with, but then add the fact that you've now drained your phone to near zero, and you can see why it hasn't taken on broad appeal.

I can pretty much guarantee that if the iphone could get its battery life to go 6 hrs in "GPS-Garmin" mode without totally wiping out the battery, it would start to dominate the entire cycling computer market. The wireless abilities alone (such as real-time GPS tracking of athletes, sending workout data to the cloud in realtime, peer-to-peer communications) would kill anything Garmin could do in a cost-efficient way.