Within 10-15 mins where I often hike the canyons, you can go from excellent cell signals (TMobile) to no service. You can even see dozens of houses and be on a paved roadway and no cell service. Go 2 miles down the road and you've got a full signal again.

I just downloaded what looks like a new great app to my HTC G2 Android phone called SOS from the Red Cross, complete with 32 videos about how to handle the usual first-aid emergencies.