I just checked McGee's _On Food and Cooking_ (highly recommended, especially if you like Good Eats), and he's uncharacteristically vague about the effect lighter and darker roasts have on caffeine content. (The 2004 edition just mentions the weight loss during different roasts, pg. 443.)
I'm pretty sure you're right, though AB has been stubbornly wrong about enough things before that I prefer to fact-check him.
Also: If your love of kitchen science extends to mad science, you might also like _Wild Fermentation_ by Sandor Ellix Katz. Making sauerkraut, sourdough, mead, beer, tempeh, miso, etc. Best cookbook, ever!
It seems the difference is very small regardless of which one has more.
I'll have to check out both books. Good Eats has inspired me more than a few times to spend a Sat. afternoon attempting to recreate what I just watched ;) It would probably help to have something a little more concrete in front of me.
Alton Brown mentions this on the Good Eats episode about coffee :)
Even then, from my additional readings on the topic the difference appears to be very small.