I'm a tried and true caffeine addict, but gave it up the day I found out I was pregnant (it tasted funny so it wasn't that difficult, but man the headache I had for the next two days!)
I read the other day in one of the many pg books I've browsed the past few weeks, that the danger caffeine poses to the developing fetus is because it keeps the baby too alert, without enough "downtime" and can therefore interfere with development.
As far as the danger to adults... I'm not sure, but we all know it's extremely addictive!