This story keeps popping up on HN and elsewhere. And the common solution I've read is "Please, just get a standup desk!" as if standing up and not moving is healthier than sitting and not moving. I've seen no study that supports that conclusion. I think the message should be: moving is healthier than not moving, which is hardly news.
Standing keeps postural musculature (those you use to keep balance upright) active. So even standing still is way more activity than simply sitting where postural muscles just do nothing. I wonder whether "active sitting" [e.g., sitting on a ball, or just sitting correctly (upright, instead of leaning on the back of the chair -- imagine sitting upright on a chair without the back)] has the same negative effect.
I sit on a yoga ball when I'm at the computer. It's a dramatic improvement over sitting on a stationary object or using a standing desk. The constant need to balance has strengthened my core and the freedom to make the tiniest adjustment means I'm always comfortable and less likely to experience fatigue. Sitting in any kind of chair feels weird now. It's such a different experience, I can't believe it would have a similar negative effect. I wish I had started years earlier, but like any activity, it's important to take regular breaks and include exercise in your lifestyle.