I've used the MotionX app with good results. The version I have lets you turn off the screen, and does a pretty decent job of auto stop-start. I have a Garmin 60csx that I hike with and the two compare favorably in terms of distance, avg, min and max spd. Elevation is better on the Garmin, but not by a huge margin. I am in Chicago, so not much elevation to speak of in the first place...
The app still does drain the battery quickly. My old 3G iPhone lasts about 70minutes from full. My way to boost that is to get those booster adapters that allow you to plug AA batts into the phone. I charge up 4 rechargeables and carry those to augment the iPhone's battery. Additionally, I turn off Wifi and Bluetooth so it is not sniffing for those constantly.