Yesterday I completed the assembly of a 35watt halogen headlight for my bicycle. To test it out, I rode the interurban trail up to Tukwila at about 5:30am today (I averaged about 18.5mph 
). It seemed about as bright as an automotive headlight, but with a little less distance from the beam. I’m powering this monster bulb with a 12volt 3.4amp hour battery. By my calculations I should get an hour and a few minutes from a full charge. Today it ran for ~50 minutes and is still shining. It seems a bit fainter, but it’s difficult to compare in daylight. If I decide I really need a longer charge, I can downgrade to a 20watt bulb.
Inspirational material (websites I stole ideas from):
http://www.vonslatt.com/headlight.shtml
http://bluecollarmtb.com/2005/07/29/build-a-helmet-light
Some todo items:
1) upload photos of the lamp housing DONE
2) make a water proof battery enclosure (The battery is shoved into a bottle cage with transparent duct tape over the wire contacts) DONE – ugly plastic bag
3) setup a quick disconnect to remove the nearly 3lbs. battery when I don’t need it
