I have the 8GB iPhone. The best app that I have found is called "Ballisitic: Field Tactical Edition" (Ballistic: FTE). It costs some money (I think $20), but it is well worth the $$ that you pay. It gives an output precise enough to get me onto a coyote at 942 yards with one shot, and milk jugs out to 800 yards with every shot (when I judge the wind and mirage correctly).
It is very user-friendly, uses the JBM ballistic model, and even takes into account things like the current local atmospheric conditions (by using the GPS and the local weather network info), spin drift, and even the Coriolis effect.
If you're serious about long-range shooting, buy it, and don't look back.