Reported in the American Journal of Epidemiology 2010; 172: 419-429 (Patel A et al) a relationship between how long you sit for each day and your likelihood of death.
This is regardless of how much exercise you do (ie. if you sit for a long time then hop up and do exercise the sitting is still a danger).
It was a big study (123000 people over 14 years).
Sitting for 6 hours a day linked to increased risk of death compared to sitting for less than 3 hours a day.
For women the risk of death increased 34%. For men it increased 17%