Wahoo Fitness has a dongle that you can plug into your iPhone 4/4s that will enable you to pair up with any ANT+ sensor. As long as the sensor full supports ANT+, your iPhone can pair up and read the data. I also believe that some Android phones comes with the ANT+ receiver built-in, so you can go that route too. The dongle is not limited to Wahoo Fitness apps either, any app that supports ANT+ sensors can use the dongle, such as Cyclemeter, Strava, MapMyRide, etc.
The basic Sensors that use ANT+ protocol are, speed, speed and cadence, heart-rate, and power meters.