I've read a few of the other posts, most of which are very agains sand. When this topic came up on a veterinary only forum on another site, there was a barage of X-ray photos showing the tonnes of animals that are ill from impaction. Certain materials are definately more prone to clumping than others, but all particulate matter is a risk. Impaction is definately a very real concern.
Now the flip side is that naturalistic vivarium are also very beneficial to leopard gecko's, and herps in general. Not necessary, but beneficial. All in all, in a top notch husbandry setup, and with healthy, adult geckos, impactions are much less common. Ill, dehydrated, parasitized, are other (even slightly) compromised individuals will not be working 100% efficiently and increasing the odds of impaction.
Whether you are a fan of the boy in the bubble syndrome for your gecko's or not, it's all good. Just make sure your setup is well researched, and that the research never ends. I use a sandy, dirt, gravel mix that resembles the dry scrubland that they are native too. Most of it is dry/ packed hard with a minimal loose top layer. I acknowledge that the risk of impaction is present. The number one symptom of blockage is anorexia, but may also include a painfull or distended abdomen, weight loss, vomiting/regurgitation, lack of stool ect. If your gecko does get blocked, it will die shortly if you are not prepared to spend a good amount of money on a surgury. If this is the case, still take it to the veterinarian for a humane euthanasia.
Well, I think I"ve presented both sides of the sand arguement and if I recieve some heat for it, well, so be it 
Ian
ps "top notch husbandry" as mentioned is not a 10gallon with calci-sand with a warm and cool end. Detail, detail, detail is all I have to say on that 