Research from the Lancet as discussed in the National Post. (Thanks Dan)
"A growing body of evidence shows the cumulative effects of all that sitting are not easily remediable by exercise. For example, a 2010 study of more than 100,000 adults in the American Journal of Epidemiology [[external] PDF] found those who sat for more than six hours a day were more likely to die than those who sat for less than three, even if the former exercised regularly and the latter did no exercise at all. An earlier study of 17,000 Canadians (“Sitting time and mortality from all causes, cardiovascular disease, and cancer”) found much the same."