I've just finished a project using an arduino and some sparkfun led matrices. I also added an accelerometer to to sense breaking and turn the break lights on.
Here is a quick video, that shows it off:
http://wyoinnovation.blogspot.com. Source code and schematic included.
Justin
I'm going nuts! This is the second bike project that has completed lately and I'm still neck deep in mine! Talk about motivation. 
I love the LED matrices. Looks like they provide a good long-distance visible area - and RGB at that. I wanted to do a tail light for my project, but all I could think of were the same-old reflector like lenses and a few high-brightness LEDs pointed in all directions.
Where did you get that clear case? Is it waterproof?
Nice work!
Thanks, the front case is an "Otter Box", the rear is a "Pelican Box 1030". They both are waterproof.
What are you working on?
Justin
I'm building an intelligent headlight system driven by a home-made alternator. I'm figuring it will have as much other stuff as I can cram into it before I finish. It will probably become a complete dashboard with all kinds of useful (useless) indicators.
http://www.chesterfamily.org/Projects/BikeGenerator/V2ControlSystem/
http://www.chesterfamily.org/Projects/BikeGenerator/
Yes, I've looked at this site before. Very cool, that is an in-depth project. What is the dash going to look like?
Justin
I haven't gotten as far as designing the dash. I only recently decided to go beyond just a headlight system, which was the result of seeing the other bike projects posted around here. But I'm thinking of maybe a cluster of two- or four circles made from PVC/ABS pipe. Or maybe a wide, thin rectangular bar . Depends on what I can find for digital displays and indicators.