Venkat Srinivasan over at GigaOm's Earth3Tech wrote a detailed entry today explaining why we've chosen lithium to power our devices.
In the context of our family trade it's a fascinating read. My old man designs formulas and plants to make industrial lubricants for a living; stuff for manufacturing, power plants and the like. Turns out he estimates the cost of his business has increased fivefold in the last decade or so, primarily because lithium is an active ingredient and it's now being used in so many other places now!
I'm still waiting for my supercapacitor that would weigh nothing, hold a huge amount of energy, would take a fraction of a second to recharge, and would blow out my whole neighbourhood's electical grid in the process. That'd be schweet.