Not much research has been done, but the most recent (2008) study concluded:
"This study suggests that ergonomically designed kneeling chairs set at +20 degrees inclination do maintain standing lumbar curvature to a greater extent than sitting on a standard computer chair with an overall mean difference of 7.633 degrees . Further research with a greater number of subjects and on different chair designs is warranted."
"This study suggests that ergonomically designed kneeling chairs set at +20 degrees inclination do maintain standing lumbar curvature to a greater extent than sitting on a standard computer chair with an overall mean difference of 7.633 degrees . Further research with a greater number of subjects and on different chair designs is warranted."
http://www.ncbi.nlm.nih.gov/pubmed/18810008