Carol's pick:
Anders, Mark. American Council on Exercise, ACE yoga study. 

Quote from the article: "For the average person off-the-street who doesn't have a fitness program, this study shows what would happen if you did eight weeks of Hatha yoga. . . .And good things would happen indeed--including increased strength and endurance, improved balance, and of course, better flexibility." 

Other research:
Though not yet available as a reviewed article on PubMed, a Science Daily article reports that work by Dr. Jinsup Song and Marian Garfinkel showed that at the end of a nine-week program, 24 women over age 65 had a faster stride, increased flexibility and improved single-leg balance and increased confidence in walking and balance. Song was quoted as saying that the research team "worked to create a very basic regimen that taught participants proper ways to breathe, stand and pose."