Check out this recent American Journal of Psychiatry Editorial (Cornell) for a more neurobiologically-accurate view of attention problems. It's certainly more on the mark than the old simplistic "hypofrontal" view.
The authors make the point that "Individuals with ADHD do not have a specific brain lesion." Rather, attention is a dynamic process involving coordination actions between multiple brain areas like the parietal cortex, cerebellum, basal ganglia, and prefrontal cortex. Casey and Durston also make the important observation that, "Given that cognitive abilities most affected in ADHD show large developmental and in dividual differences, it may seem perplexing that functional imaging studies of ADHD have not emphasized tracking changes longitudinally within individuals."
This model is more on track with brain science. We also shouldn't forget that this cognitive control / attentional network undergoes tremendous developmental change from the preschool years into adulthood, and it is in general both plastic (changeable) and highly trainable.
From Behavior to Cognition to the Brain and Back: What Have We Learned From Functional Imaging Studies of Attention Deficit Hyperactivity Disorder? pdf