Thanks for the support, lads, and don't be too hard on Clueless--I can see where my wording might have led him wrong.
When I say that the other foot should be back, with the knee bent, I don't mean that that foot should be in the air! If that's how Clueless read it, he's right: that would be almost impossible to hold in a set position.
What I meant was that the rear foot should be *on the track* with the rear leg somewhat bent at the knee. In fact, the ball of the rear foot is firmly grounded; remember, the rear foot makes the first push, though the push you notice is the one from the front leg. (Sprinters do the same thing in their starting blocks, by the way: very brief, sharp push from the rear foot *followed* by a longer push off the front block. This is why you coach sprinters to firmly load both feet into the blocks...)
As is so often true in running, a useful teaching tip is to have the runner (in the set position) focus on the *backward* movement of that front elbow when the gun goes off. As that elbow zooms back, the rear leg steps through and you're on your way.
As to its feeling "natural" to have the same-side arm and leg forward--I know that's true for some people, but with a bit of practice the opposite arm/leg start feels just as natural, and better balanced to boot. But you do have a choice of starting positions; by contrast, you don't get to *choose* same-side-or-opposite when you're actually running--nobody (except the guys on a Greek urn) can run same-side. So it makes sense to choose the starting position (opposite arm/leg forward) that best leads to your running position!
Now, to add a bit of fuel to the fire: You get a few seconds of alactic anaerobic running at the start of any race, during which you get to run essentially as fast as you want with no lactic acid production. So if you're coaching a sound start, you also want to coach the athlete to get up to speed immediately by really sprinting off the line. Now, it may only take four to six steps for the distance runner to get up to his race pace, and I'd be very wary of coaching him to sprint farther than that--the temptation might be to blow out the first 200 and build a huge oxygen deficit--but in theory he could get maybe 30m with no lactic acid penalty.
In any case, I never wanted a runner--even in a 10k--to "walk" off the starting line. (Even pace is not the same as even effort!) If you go to the line with the mindset of a sound start, it feeds into a sense of controlling your own race--of taking charge of your own pace, right from the gun. And I think a take-charge attitude is key to one's best competitive effort.