My (Chris) coffee journey began at Starbucks many moons ago and I remember blowing people's minds with the idea that Breakfast Blend had more caffeine than the dark roast. And I was so proud to have this insider knowledge about caffeine content. However, I have recently learned that my little factoid about caffeine "burning out" of coffee as it's roasted is yet another victim of some myth busting.
Check out this article on the subject at KickingHorse Coffee.
It turns out that caffeine is fairly stable during the roasting process so that caffeine diminishes very little as the bean darkens and dehydrates.
The Robusta species of coffee can have as much as twice the caffeine content as the Specialty-grade Arabica beans. This of course causes the increased bitterness of Robusta vs. the tasty acidity and sweetness of Arabica coffee.
So if you are consuming coffee purely for the caffeine effect, don't worry about whether it's light or dark. The difference is negligible.