Found this pretty good site about lights/side markers/tail lights, etc about the laws. Whats great is the guy has even pulled some of the intepretations from the NHTSA legal interpretations database for you.. so you don't have to

Its a pretty good read.. and leaves you a good understanding of why certain markers are legal or others aren't.. etc as well as links to the straight up US laws, etc.

Excellent Site...

Long live google!

http://fmvss108.tripod.com/index.htm