One part of the 'more healthy' claim I've read about is that you're bound to move a lot more if you're standing. You'll shift weight more. You might wander off for a second, thinking. It comes more natural instead of lifting your rear out of a comfy chair.
Of course it can't be healthy to sit 8 hours straight without some pause for walking or stretching. Combine that with some exercises 2-3 times a week and you would eliminate almost all of the health problems related with sitting.
Why is that obviously bad? I stand for 9-10 hours straight. It took probably two weeks to get up to that point, but I rarely even sit down when taking a lunch break. I don't use a gel pad and I stand barefoot at home, or in some form of barefoot (vivo's to look nice) when I worked in a office. I almost constantly shift my weight. And every 10-20 minutes I do calf raises, stand on one foot, do free squats, something to move some blood. I'm still standing in all of those exercises though and most are just second nature. I stand still while typing, but move quite a bit when I'm reading.
What about period when people sat 40 hours a week at the desk? It didn't happen either.
To answer your question I think that sitting 8 hours every day is not very healthy, but it's far better than standing 8 hours. Break long period of sitting with some walking, stretching or some light exercise, and you would eliminate almost all negative effects of prolonged sitting.