On top a good case, you might also want to consider using an external GPS with your iPad on the farm.  

A couple of companies makes them from Garmin GLO, Dual, Bad-elf, and a few others.  (cost about $99) 
They are mostly all now bluetooth since the cable ends keep changing.
They will increase the resolution of the iPad from 10m (3G/4G version) or 100m (wifi only version) to 1-2m.

There is also a series of cable that are NEMA compatible coming that will let you repeat your RTK signal into the iPad as well.  
Heard they are about $250/each and expected in Jan 2013.  Will pass the link once I find it again.  

Other thing you might want to consider is one of the HD screen polarizing screen covers, so it works better in direct sun light.  Apple store sales some for like $15.

We have several hundred iCropTrak customers with iPad in the field and they seem to love the iPads large screen and more intuative touch programs.
We only heard about a few (less than 10) broken iPads in past 2+ years and only one of those happened in the field (it went swiming). 

Just remember like your tractor you need to get all the right gear for them to provide the most benefits possible.