I would suspect that for some it is the caffeine that is present in
chocolate. (Though at the same time, if this is true, I'd also suspect they
would blame caffeine in coffee, tea, colas, energy drinks, etc. too).

We have recently put together an article on caffeine and chocolate that goes
into a fair amount of detail about how much caffeine is found in
chocolate.(
http://www.amanochocolate.com/articl...chocolate.html )

Hopefully, they will get to the bottom of this soon. I have a few friends
who can not eat chocolate because they are prone to get migranes afterwards.
It would be nice if they could come up with something like "Beano" but for
those who want to avoid the chocolate migranes. ;-)

-Art