This http://image.sportsmansguide.com/dimage/116692_ts.JPG?cell=200,200&cvt=jpeg is how I believe it is SUPPOSED to be worn (?), but I read that you could put the chest strap back behind your neck to conceal it, then I watched "Opperation Dumbo Drop" this morning, they were all wearing them with the right strap over the right shoulder and down to the belt. Wich was the way they were actually used by the military? All of the above?

I spent 23 years in the military 15 of them in Armor, both M60A3 and M1's we almost always wore them as in the photograph.
Remember its not about concealed carry in the military but keeping the weapon out of the way while performing your main mission / duty.
In tanks if your using either your pistol or rifle things have already gone way too wrong....(why use a rifle or pistol when you have a 50, 2 30's, or a 120?)

On the other hand in an urban environment as a TC, it can be a lot faster to grab your M4 or M9 than to slew the cupola, Ask some of our Iraq vets...

I sometimes use an M7 (was my dads) over here in Iraq now (inside the wire) and use it like the photo you posted (except for the belt loop). The ACUs cover the belt so it would be uncomfortable.