Flash can be harmful in exceptional circumstances:
Newborn babies - avoid flash for the first few weeks. Especially full power with larger flash units.
People with epilepsy. I've not heard of any preflash incidents triggering an epilectic fit but there's always a first time, especially if you are using the strobe function on some of the more advanced flash units.
Full power direct in to the eye at extremely close range: This *may* lead to damage of the retina, especially with very high powered units combined with flash extenders.