I have 12V lighting, and I'm not alone. It's great for me as it makes the lights themselves cheaper, and it's only one battery (a big one for me, as a commuter, 10 amp-hours) and all I do is plug in when I get home at night and unplug in the morning. Most 12V bike lights are DIY I think, so you find the descriptions in the total geekiness thread instead of here.
For headlights, I prefer MR-16 bulbs over direct automotive use as shown in the links as the current draw on the automotive ones is usually too much for me, even in LED headlights.
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Longbikes Slipstream