For one thing, battery life may be a concern on an all day hike. You may have to get one of those AA recharger adapters to bring along. Secondly, the phone isn't nearly as rugged as a stand alone unit dsigned for hiking would be. Dropping the phone or getting it wet isn't recommended at all, where a good stand alone may be able to handle more accidental abuse.

That being said, you may want to check out geocaching.com for software reviews that would apply...