Just like athletic ability, it's probably some of both. The "nature" part is a physical tendency toward the symptoms (for example, a tendency toward inhibited dopamine uptake), and the "nurture" part might either reinforce an actual condition, or introduce a similar sort of behavior in those not physically pre-disposed toward it.
I know kids who fall fully on either side, where nature or nurture are probably completely dominant. I also know kids whose cases are somewhere in the middle. For example, with our daughter (who is very bright) we find that we can manage things by behavioral things (ours and hers), and by diet (we have to keep her away from red dyes). As parents we can guiltily point to things we could have done differently when she was little. But there's undoubtedly a strong "nature" component with her: she's pretty much exhibited the tendencies since birth.
I am quite certain that ADHD is too-often used as a convenient excuse. But that doesn't mean it's fake.
As I said at the outset, there's a powerful ideological component to the ADHD debate, as can be seen in this very thread. I have to say that those folks are probably as damaging to kids as are those who over-subscribe to ADHD diagnoses.
Not only do they loudly shout out piles of misinformation (i.e., "there's no such thing"), but they also make those on the "pro-ADHD" side appear to be calm and reasonable -- thereby strengthening the very thing they seek to get rid of. (Sean Hannity made a complete ass of himself on Friday with just such a claim, when he was discussing this article.)
I have no use for them.
I only have one child, a 4 year old boy who is an angel and can clean up his room for 20 minutes without losing focus. So my anecdotal experience biases me. When he won't pay attention, I just make sure his choice produces a miserable universe for him and rely on his innate intelligence and self-interest to take hold.
It is hard for me not to notice that all the children in my church who are diagnosed with ADD also have parents I consider shockingly lazy in their parenting.
I can't make an argument for the whole world based on my amateur and anecdotal experience, but it is hard for me not to notice the pattern.