The text, in case that link dies:

"No one wants to be a slouch, but it turns out that might just be what's best for your back if you're bound to a desk all day. That's right: Although you've been admonished all your life to "sit up straight," the healthiest posture seems to be considerably more laid-back - 135 degrees back, in fact. When researchers in Scotland and Canada scanned the backs of 22 healthy men and women as they assumed three sitting postures - sitting straight at a 90-degree angle, hunching forward, and leaning back 135 degrees - they were surprised to find that the spine was actually least stressed in the laid-back position. The worst? Sitting ramrod-straight created the most disc movement and strain on the spine.

"Sitting unsupported puts more pressure and stress on your lumbar spine than standing," says Scott L Blumenthal, M.D., a spine surgeon at the Texas Back Institute. Relieve the pressure by sitting in a chair with good lumbar support, so it's easy to maintain the natural curvature in your lower back."

Source: March 2009 issue of Bicycle Magazine. I think this is covered by fair-use.