The impact of movement — even leisurely movement — can be profound. The muscle activity needed for standing and other movement seems to trigger important processes related to the breakdown of fats and sugars within the body. When you sit, these processes stall — and your health risks increase. When you're standing or actively moving, you kick the processes back into action.
I've started keeping my digital exercise watch next to me programmed to beep at 30 minute intervals when I am working on the computer or sitting to remind me to get up and move around. I think it's a good practice one should get in the habit doing.