Dr. Gabe Mirkin has an interesting post on the putative dangers of sitting. He argues that sitting may not be dangerous for vigorous exercisers:
“People who exercise intensely need to spend a lot of time sitting or lying down because muscles are damaged by intense exercise. If vigorous exercisers and athletes do not rest, their muscles do not heal from their previous intense workouts, they will be unable to train intensely and they are at high risk for injuries.”
Mirkin is arguing that the studies of sitting have not taken sufficient account of differences in the amount of exercise. In my own field of educational psychology, I am aware that a lot of research ignores the potential confounding effects of individual differences.
My own experience suggests that prolonged sitting does tend to reduce flexibility, especially in the hamstrings. Until this issue is clarified I will continue my current practice of taking five minute standing breaks for every 25 minutes of sitting.
Here is a video on good posture from yoga teacher Larry Terkel:
peakmemory 
We’re going to have a real problem when standing becomes unhealthy too!