Back pain is the most common complaint of office workers in the US. And go figure, the problem might just be their perfect posture.
In a recent study, 22 healthy volunteers had their spines scanned with an MRI while sitting in 3 different positions. The position causing the least amount of undue stress on the vertebral disks was a relaxed position, with 135 degrees between the top of the thighs and the spine. Sitting upright, with the excellent posture our mothers and/or ballet teachers drilled into our heads, created the most stress on the vertebral disks, even more than the position of having a 70 degree angle between the legs and spine. I would have to argue though, that slumping forward with a rounded spine is worse than sitting upright. Slouching forward stretches the ligaments that structurally support the spine and keeps the muscles that should support the spine in a stretched position so they can not functionally help stabilize your vertebra.
This image from a BBC News article illustrates the positions best. Do note: In all positions the participants have back support and both feet on the ground (just in case you want to experiment with your work station).
Fit's Two Cents: I think that we just aren't really made to sit 10 hours a day like many of us do. We evolved as a species to be hunter-gatherers, which means walking around a lot and squatting (no chairs forcing our spines into a 90 degree angle from our legs) when we needed to take a load off our spines. So walking around as much as you can during the day is always a good idea. Your back will love it, and chances are, your mind will to.