Chronic computer use may be deadly, Business Journal reports. A New Zealand man who spent about 18 hrs/day in front of his computer developed a blood clot and almost died of pulmonary embolism.
Incidentally, in the US over half a million cases of pulmonary embolism occur each year; it is fatal in about 10% of the cases. The cause, in other words, may have been something entirely independent of sitting still for prolonged periods of time.
Nevertheless, you might want to get up and stretch once in a while.