It certainly can't be taken as a given, though, that if long sedentary periods are harmful, long periods of standing still are significantly better.
Before I started standing, I'd go out for lunch and come back around 1:00 or so. About 30 minutes to an hour later, I'd be fighting off hibernation. My post-eating-sleep-reflex thing was just absolutely overpowering. Even now, having to sit down for a meeting around that time of day has me close to nodding off.
When I stood, this was completely eliminated. I'm not a doctor, so I couldn't tell you why, but I am a lot more consistently alert/energetic while standing. This has been the big win for me over everything else.
http://www.tastebook.com/recipes/3016050-Flaxseed-Wrap?full_...
For me, that's actually the critical difference.
Details: http://www.reddit.com/r/raldi/comments/kfjet/cornells_standi...
I'll dispense with the full Bayesian treatment here, but what we have here is a case of the base rate fallacy (although not the form that is usually discussed): failing to account for the prior probability of the evidence. That is, we have some evidence, which is that the anecdotes we see are largely in favor of standing desks being helpful, and we need to account for how likely that evidence is regardless of whether standing desks are helpful.
Unfortunately, as our friends with the magnetic bracelets demonstrate, that probability turns out to be very high. If something works, we see overwhelmingly positive anecdotes. If it doesn't work, we see overwhelmingly positive anecdotes.
Why might this be? Well, for one thing, there is sensitivity to placebo effects, because have purely qualitative phenomena that are being reported only subjectively.
But even stronger is a selection bias that has to do with people's behavior in reporting and reading. Someone is unlikely to write about their experiences publicly who tries a standing desk for a while, finds no significant benefit, and goes back to a sitting desk. She won't find it interesting to talk about, and she may also be self-conscious that she didn't do it right, and would be inviting criticism. After all "I tried X and it didn't work" is, on some level, an admission of personal failure, and people aren't particularly prone to those. And if she does write it up, her write-up isn't likely to get all that much attention because "X didn't work for me" isn't a particularly interesting story.
But one who finds the standing desk to be a miracle cure for all of her problems is relatively likely to write about it, and people will take notice, because "making a simple lifestyle change improved my quality of life" is an attention getter.
For my part, my week with a standing desk just led to upper back pain (which is not unusual for me, but I am generally able to avoid).
Physically, this works out okay for mousing tasks, since those are one handed and offer a large degree of freedom. However, it just doesn't work for two handed tasks, like programming or writing, which anchor both of your arms, because your arms are critical for balance and, thus, movement.
I also found that this desperation to move around while standing affected me mentally. That is, when standing, I was fine doing short, low-attention tasks like responding to email or IM, since I coud move a little between each task. However, for longer tasks (again, like programming or writing) I found this to be maddening. I felt as if I was constantly being tugged and distracted by this urge to move.