Standing is great, but walking is even better. I've been using a treadmill desk for the past 7 months and have seen the same benefits Jamis noted - less fatigue, better concentration, improved posture. Walking at a slow pace (.7 to 1.2 mph) is less stressful to your joints than standing still, but you don't break a sweat and can type, use a trackball, and talk on the phone comfortably.