Okay, I'm going to take a stab at this one. I'm so sick of reading this stuff...
Time and time again I read threads here that say nobody heel strikes naturally, watch little kids run--they never heel strike, shoes cause heel striking, run barefoot and you won't heel strike because it is painful or it feels unnatural...
I have two kids under the age of six. I have been watching, videotaping, and photographing them running both in bare feet and in shoes for several years--basically as long as they have been able to run. Barefoot or not, my daughter is a midfoot striker, primarily because her foot lands under her hip, directly under her center of gravity. Barefoot or not, my son is a heel striker, primarily because he leans forward over his hips and overstrides, causing his foot to strike well in front of his center of gravity.
It's a mechanical process of learned muscle movements. The fact is that some people (the majority) are born to heel strike, and some people are not. I've observed a small percentage of people who couldn't heel strike if their lives depended on it, and I've met a large number of people who couldn't midfoot or toe strike for a 30 minute run without getting injured.
Also, everyone's ability to get closer to a midfoot or forefoot strike is highly dependent on the speed they are able to sustain. The faster you run, the more you tend to plant under your center of gravity and therefore have a mid to forefoot strike. If your foot lands directly under your center of gravity, it is impossible to heel strike without actively and purposefully dorsiflexing, and lifting your toes so your heel lands first.
Okay, I'm done with my rant. Nothing personal, just sick of seeing this over and over.