http://ergo.human.cornell.edu/CUESitStand.html
Being stationary in either a sitting or standing position is bad. There are ways in which sitting is uniquely bad, but there are also ways in which standing is uniquely bad. Both are bad. The answer is: you need to walk around or in some other way incur gross motor exercise regularly, on the order of twice an hour or so.
This has been one of 'tzs beats on HN for the past year; he's written some pretty detailed comments, try searching at the bottom of this page for [standing by:tzs].
http://www.nydailynews.com/life-style/health/sitting-2-years...
And a year ago:
"The study found that women who sit for more than six hours a day were about 40 percent more likely to die during the study than women who sat fewer than three hours per day."
http://www.healthkicker.com/751136264/sitting-for-hours-may-...
http://ergo.human.cornell.edu/CUESitStand.html#
http://www.washingtonpost.com/wp-dyn/content/article/2010/10...
I've got a timer on my iPhone, two different spots in the office to sit (my desk and one of the couches in the lobby), and no weird gadgets; the Cornell page seems to suggest that this is pretty optional, and I dont' have to pronounce any weird Ikea furniture names to describe it. :)
One of the potential reasons behind both of these is that being sedentary in any position is essentially bad (in one way or another - a PhD out of Australia whose name escapes me has done the research here).
The body was built to move, giving extra options seems like a win and puts less stress on any one system (esp. musculo-skeletal).
A recent study reports a reduction of plasma volume by 500-550ml or about 16% after only 15 minutes of quiet standing. The reduction in plasma volume in turn triggers several changes that can be grouped into hemodynamic and humoral effects, including but not limited to increased hemoconcentration, increased release of catecholamines, changes in pulse pressure, and increased heart rate. For example, one recent study measured a significant increase in heart rate and catecholamines after 10 minutes of standing for men with CVD and also for healthy referents. (em mine)
Increases catecholamines = increased stress.