I don't have a good hard cover for mine yet, just a soft silicone one, and a leather belt holster. The holster keeps it reasonably well protected from falls, but of course you can't use it while it's in the holster.
I'm not sure if there's anything really weatherproof for the iPhone, although the
Otterbox products are well regarded in this area (at least for other devices).
For weather proofing while bushwalking with the iPhone (not that I always take a phone bushwalking), I just put it in a very small zip-lock bag. It's fully functional without ever taking it out of the bag. You can still hear it and talk into it, and the touch screen still works (it's really a proximity-sensitive screen, not a touch screen). While in the zip lock back I can insert it and remove it from the holster too (bag and all).
I did discover though, that the touch screen requires something specific about the finger tips to work. It won't work with gloves on, and it won't work with my nose. Very odd. Maybe it's temperature-related (it was a cold day - hence the gloves).