A study published in the April 2004 edition of Pediatrics identifies a correlation between watching TV by children below the age of 3 with ADHD in later life. Carried out by Dr Christakis at the Department of Pediatrics, University of Washington, Seattle, Washington, USA, it used longitudinal data on over 1200 children to compare early TV viewing habits with educational problems in later life. It found a 9% increase in risk of attention problems for each daily hour of television-watching. 
It should be noted that this is a statistical correlation that cannot prove cause and effect. As the study notes:  "… we cannot draw causal inferences from these associations. It could be that attentional problems lead to television viewing rather than vice versa. . . .It is also possible … parents who were distracted, neglectful, or otherwise preoccupied might have allowed their children to watch excessive amounts of television in addition to having created a household environment that promoted the development of attentional problems."
Abstract in the journal Pediatrics. Coverage at WhiteDot.org. A PDF of an interview with Dr Christakis, lead researcher. Also the link between childhood TV and poor adult health.