It is true.... I think if you read through this forum there is somewhere a whole discussion on this. Basically caffeine is burnt off the further you progress through the roast process. Caffeine levels in the cup are also affected by the brewing process.
There are a couple of interesting articles you might want to look at-
The dark roasts I drink always seem to have plenty of caffeine. The caffeine also seems easier to taste through the coffee. The extremely dark, charcoal-tasting roasts however do seem to have noticeably less caffeine, and even I added some crushed caffeine to them like I've done to coffees that don't have enough I would still strongly dislike their taste.
Caffeine does burn off with roasting, but so do other things. Interesting that if you measure caffeine by weight in dark roasts, you get more caffeine than in light roasts. Moisture evaporates faster than caffeine burns out. If you measure by volume, you get less because when you roast beans, they expand.
I've heard a number of people say they can taste caffeine in Starbucks. Caffeine is extremely bitter - anyone who has chewed a NoDoze can tell you that.
But is the bitterness in Starbucks really the caffeine or is it over-roasting and rancid oils?
"javahill" thank you for clarifying this, to be honest, I was of the crowd that just said darker has less caffine knowing you are dealing in the actual volume of grinds that fill the portafilter. You have taken a scientific approach to this and I have to give you a thumbs up on your analisis and you are very much right on.