INFJ's are not really all that spontenious, as our actions are mostly planned out. While yes, I do like stimilulation as much as I can get, I can survive without it. I would venture to guess most INFJ's are the same way.
Spontaneity has little to nothing in common with ADHD, as hyperactivity does not equate to spontaneity. Fidgeting and squirming, talking out of turn, and generally not being able to sit still aren't the same thing as, say, randomly deciding to go skydiving (extreme example but hopefully this illustrates my point).
I'm seeing a psychiatrist now for a possible ADD (ADHD-I, whatever - the people writing the DSM sure do like to spark confusion) diagnosis. I've only been to one appointment, but after talking for about three hours he told me that I fit almost no criteria for ADHD, but that there is substantial evidence for ADD. I don't change body positions or constantly tap my toes (well, not in the meeting), but my mind wanders a lot and I usually can't sit down long enough to make any headway studying. My brother and dad both exhibit symptoms (my dad being an INFJ, my brother I don't know..), and ADD is supposed to run in the family.
Personally, I think ADD (not necessarily ADHD) is a copout. If anything, it's a product of too much stimulation in this day and age. I wonder how people
don't get distracted, being constantly surround by cell phones, computers, television, etc. I'm willing to swallow my pride, though, if I can get something to help me pass my classes.
To answer the original post, though, I've seen the topic brought up before, and I doubt having INFJ tendencies has much correlation with ADD/ADHD. If INFJ's do tend to have ADD/ADHD more than other types, I think that Minerva's post makes a lot of sense. I feel like I'm naturally interested in a lot of different things, and it's just difficult to focus on ONE neat thing over all the others. I don't like the idea of medicating ADHD kids just to shut them up, either. Giving amphetamines to children seems lazy and irresponsible to me. Coaching kids on how to act in school is what elementary school counselors are for.