Disclaimer: I don't take ZMA, I take magnesium and zinc separately...probably because I'm taking a ton of magnesium and just a little zinc. Also I have a definite pro-magnesium bias because when I started taking it, I saw noticeable symptoms of what I had suspected was magnesium depletion disappearing with sleep, energy, stress levels, and a bunch of other things.
I haven't ever read anything about too much magnesium decreasing calcium absorption. It's actually the other way around: calcium cannot enter the bones without adequate magnesium. If you get too much calcium it pulls magnesium out. Other things that deplete magnesium include diuretics, chronic pain, various diseases, alcohol, drugs, tobacco, caffeine, stress and crappy diets (too much calcium, carbs, sugar, sodium, etc.)
Most people are deficient in magnesium. The lowest estimate I've read was 70% in the US and the highest was 90-95.
This article is pretty good and cites a ton of studies, and there's sections on athletes down near the bottom: http://www.mgwater.com/conseq.shtml