I ordered a yoga ball from Amazon a while back to help with my posture and lower back pain and only recently decided to start using it, especially in light of knowing about things like this.
I gotta say at this point, I recommend getting one and using it. I've been sitting on the yoga ball instead of my computer chair or couch for about a week now, and I don't mind it too much at all. It does take a little effort, as it is nice to slump down on a nice comfy chair, but I don't wind up feeling like a slug after a while or all stiff when I get up.
Also, it is fun. There are about a zillion different ways to sit on it; feet in front of you on the ground, feet in front of you not on the ground, balancing on it cross-legged, kneeling on it, straddling it, even standing on it. All the while, different core and leg muscles are being activated and strengthened with each position. The act of staying balanced alone constantly works many muscles that a static position would be allowing to atrophy.
For instance if I am watching a TV show I might see how long I can balance in an unsupported position. I spend a few minutes standing on it (with something close by to grab if I lose my balance) and doing squats just for the hell of it. I think of more creative ways to use the thing every day. Today I realized that if I balanced on it on all fours, I could slowly move around on the floor by rolling it around beneath me as I reposition my hands and knees. This provided a very interesting workout.
So I guess I'm going to go ahead and call myself a non-sitter now, at least when I'm at home.