a "kneeling chair" (http://en.wikipedia.org/wiki/Kneeling_chair) is a similar solution that keeps your spine straight but doesnt put stress on your legs. i have a friend who has one and he loves it
The central problem with kneeling chairs is that one of the most important ergonomic principles is that you have a natural (and necessary) propensity to shift positions every few hours. Kneeling chairs don't allow you a range of positions the way that conventional chairs, exercise balls, standing, etc. do. The ergonomics people I knew always went out of their way to say kneeling chairs were actually a bad idea.
Having said that, whatever works for you, works for you. But for people considering a kneeling chair, you want to be careful not to just try it in the store for 10 minutes, then buy it.
But now I'm thinking standing or walking is the way to go. There is deep evidence that walking while working improves performance ... and is more fun.