Here’s an interesting thread on fear of heights at TrailForums. One extract:

I had a hiking accident in November 2003, falling down a very steep section of trail on the AT in the Whites while solo hiking. I spent two days lying there by myself until I was able to hike 10 miles to safety. I had 7 fractured ribs and a hemothorax, spent a week in the hospital. The following summer I hiked 1170 miles of the AT, but ended up leaving the trail because I was getting retraumatized and becoming increasingly afraid that I’d fall again. … I think that dealing with the aftermath of trauma is very complicated and not as simple as “just do it” and get back on the trail.

I wouldn’t say hiking has cured my fear of heights, but I don’t seem to get freaked out by steep sections of trail. I’ve gotten to places where I’ve simply turned back because it looked too dangerous, but it wasn’t fear of heights that bothered me. It was fear of falling, or, more precisely, a hard landing.