In Vietnam the most prevalent item on a GI's dog tag chain was the can opener for his C-ration can. This "fold flat" piece of heavy gage sheet metal, called the P-38, was as ubiquitous as the dog tags themselves. With the advent of MRE's, the need to have your P-38 ready at hand was no longer critical.
Fast forward thirty some odd years. The Army now has computers doing an amazing amount of work in the field. And this is especially true in Iraq. One things soldiers did discover was that the standard 3.5 inch "floppy disk" could not stand the extreme of heat and dust found in the Iraq or Afghanistan. The "memory stick" or USB memory card, soon replaced the 3.5 inch disk as the portable storage device of choice. As it turns out, these "Memory Sticks" fit nicely on the dog tag chain. Where in Vietnam you would have seen a P-38 on every neck, in this war it's a memory stick. Troops keep their email from home, digital pictures and all manner of stuff on these small devices. Its only a matter of time before the brass try and forbid the practice, as you are not supposed to take such documents with you into a combat zone (lest you be captured and the data prove useful to the enemy.) William Gross
Every soldier gets issued two ID Tags and a chain hold the tags as they are worn around the neck. These "dog tags" have been with the army since 1906. Early on, soldiers found that if you need access to something important and didn't want to have to dig through your pockets to get it, you could hang it on the same chain you had your dog tags on.