Are there LED based dynamo powered lights suitable for night time MTB riding on singletrack?
i have been using the Supernova e3 Triple for night-time singletrack missions for the past two winters.....all I can say is awesome, completely awesome. 870 Lumens. bright.

I used a Shimano Alfine dynamo hub and built a complete 'night wheel' with a DT 4.2d rim, it has rubber and a rotor mounted so swap-over time is very quick. We have months of mud, ice and snow here and never once had a problem with the high quality Supernova connectors.

I cannot recomend this set-up enough. with the E3 rear light you are also nice a safe for the Asphalt connectors between the trails.

good luck.