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?