Quote Originally Posted by OakLeaf View Post
You can get an ANT+ dongle to pair an iPhone with any ANT+ compatible HR strap.

The issue with using a phone is battery life. Once you turn the GPS on, you're looking at maybe 4-5 hours. Driving a peripheral off your phone would kill the battery life even more. I like iMapMyRun for walking, but if you're going to do something other than walk for a couple of hours and then immediately plug in your phone, you could wind up incommunicado.
I used my iPhone once for iMapMyRide and I did indeed wind up up incommunicado after 3.5 hours of a 5 hour ride...I don't know if the 3Gs or the IPhone 4 would be any better but I wouldn't think so...