It seems researchers have discovered that sitting for extended periods actually reroutes the fat in the body:
"The radioactive tracer revealed that when the animals were sitting down, the fat did not remain in the blood vessels that pass through the muscles, where it could be burned. Instead, it was captured by the adipose tissue, a type of connective tissue where globules of fat are stored. That tissue is found around organs such as the kidneys, so it's not really where you want to see the fat end up. "I thought I had gained weight because of the food I was eating and the lack of regular exercise; now I'm kind of happy because I can blame my waist expansion on work, TV, riding in the car and the Internet.