I am HARD on cell phones, scratched screens, busted screens, you name it. When I got my Ipaq 910 last month, I bought an aluminum armor case for it from boxwave. Like this:
Armor Case - iPAQ 910 Metal Case / HP iPAQ 910 Metal Case / iPAQ 910 Aluminum Case / HP iPAQ 910 Aluminum Case
The doodad that goes on the back for the belt clip goes into a machine thread boss on the back, so it could easily be adapted to another type of mount. The phone is SUPER well protected as well. It is an impressive case that protects the phone from people like me who just jam it in their pocket or chuck it in the car with them.
That being said, the best option is a bluetooth headset or earbuds. Then it doesn't matter where the phone is and there is no wire to fuss with if you have to get off the tractor. If you get the earbud type, they would easily fit under over the ear muffs style hearing protection.
I use my phone as a computer, phone, mp3 player, movie player. I have used it while running the tractor, waiting on my son at baseball practice, mowing the yard, it goes everywhere with me and I have both the earbud style headphones and over the ear bluetooth headphones depending on where I am.
While I know a lot of people hate cell phones and how they are a "leash", it does the exact opposite for me. It buys me the freedom to do the things I want, more or less when I want instead of being chained to a desk or office.