I 'successfully' rigged up a 20 watt 12 volt MR-16 lamp lighting system for my commuter in Dec 06. The biggest challenge/headache for me was mounting the lamp. In the end I kludged it together using a couple of hose clamps (on the stem). I did not use a fuse. I already had a battery charger (for trickle charging my motorcycle battery), so I made sure that I wired up the same kind of connectors on the 12-volt (4.5AH rating) SLA battery. I added a toggle switch (make sure it has adequate amp rating) and that was it. I mounted the battery on top of my rack and clamped it down using a u-bracket (again from Home Depot). Anyhow, all worked well and it sure lit up the dark spots on my commute (all urban). However, I caution you on the following:

1. the whole solution does add pounds to the bike - for me it was atleast 4lbs - I am newbie so I noticed it.
2. the bulb kept popping out due to the vibration - I had to epoxy it to the socket.

I am currently looking into the following:

http://www.littlefishbicycles.com/bikelight/