This very same topic keeps coming up over and over on this and other precision ag message boards. My experience with growers and different software says that the first software company that markets a fully functional field version allowing cloud based data sync will make a killing and dominate the market. Many production Ag professionals are not comfortable with traditional pc or Mac operating systems, but love their iPads and use them daily.
What I see so far is the challenge of inputing submeter or RTK gps input into an iPad. There are Bluetooth submeter gps units out there, but most require either RS232 serial or manufacturer specific cable adapters. The internal gps in the iPad is okay for general navigation and getting you close to a management zone, but is not "precise" enough for VRA.
The first steps toward the iPad/iPhone GIS I have seen so far is ArcGIS. But this is not specific to agriculture.