heh, the dentist chair was pretty funny. i've actually thought about getting one like that, because you lay down and let gravity hold your spine nicely in place. it would be healthy, imo.
at the end of the day, the best chair is the one that helps you maintain posture and a strong core/mid-section. shoulders back, chest out, etc. whatever chair does that is best.
Re: dentist chair pic - well, lol, yeah, kinda. That's the idea anyway.
The foremost expert on spine health in the world is Stuart McGill - he argues, as I did above, that you don't want to "hold your spine" in place for long periods of time. That's the issue: you really want to be changing positions a lot.
Also, surprisingly, lying down for long periods of time is not spine healthy either. Again, according to McGill.
The best "chair" is probably one with no support at all: simply a stool.
Also, I forgot to mention that I try to sit on the floor a lot also; once you get used to it it's wonderful - for your flexibility (particularly in your hips) as well.
I'll check out Ferris' blog thingie.
cheers alL!