I remember reading somwhere that DoT's produce less threat per damage. However, reading through the threat discussion on the wow forum I can't find any mention of it. Could somebody enlighten me please?

A point of damage is a point of damage. DoT's generate just as much threat point for point as anything else, but as Emarius points out, it's spread out more so it's less noticeable. Great for offtank targets if you don't have to move to do it ;)