I think this person has found a heck of an in-between position. It's not $40, but it's a heck of a lot cheaper than a geekdesk. http://blog.melchua.com/2011/07/09/fitting-yourself-a-sit-to...
A top-of-the-line aeron is about $700; the stool version is about $1200.
Do you need an ergonomic chair?
In my experience ergonomic means with a too much bended back shape that will DISTORT(!) your natural spine shape. Unfortunately they use statistic to decide what is the ergonomic "best" shape, the problem is that people backs in the west have gotten less and less straight the same way they are getting more and more overweight.
Instead of buying an expensive chair, in my opinion it is a best investment to buy a good book or take some good posture lessons that teach you how to sit, stand or sleep properly.
Your back could support itself without effort if it is straight.
That said, I agree with you that an ergonomic chair would make little sense for a standing desk, as one will obviously sit less (only when too tired to stand).
[0] http://news.bbc.co.uk/2/hi/6187080.stm (2006)
To put it another way: every person may have a different use pattern when given the option of equal comfort at seated and standing heights.
Even without that, a detached footrest would still be rather easy to find.
Most stools let you adjust the height without problems, so you could put your feet on the floor. Some of them you could even attach to the floor, so you could use it as a support.
You don't need to. You could buy a stool and sit down when you need it or want to.
For me, it took 3 months to get the muscles necessary for standing up developed, and 6 to learn a correct posture good for my joints.
Today I use a wall as support for my back while standing while I use the computer attached to another wall with a mechanic arm(vesa tv mount with an imac adapter).
This is by far the best idea I could have, it feels totally different to work standing up.
I came to the same conclusion after switching to a standing desk for a few months.
Now I'm using a yoga ball and it's amazing. It's immediately intuitive: just sit on it and shift to another comfortable position if you've been in one place too long. My core is even getting stronger from balancing (although it doesn't feel like a workout). I have a 65cm one for desk-height and a 75cm one for counter-height. I highly recommend it.
I find a stool not really a valuable fatigue reducer compared to a real chair. I couldn't handle standing all day, and the stool was better, but not great. I finally bit the bullet and went fully adjustable with a real chair, and it's so worth it.