In the weight training community, yoga is seen as problematic because it involves a lot of spinal flexion (ie bending the spine forward). This is seen as a general no-no (which is why sit-ups are generally viewed as bad as well today).

Stuart McGill is often quoted in these matters, as he has done extensive studies on the mechanics of the spine.

http://www.dailymail.co.uk/health/article-440309/Is-Pilates-...

http://www.amazon.com/Back-Disorders-Second-Stuart-McGill/dp...

http://www.t-nation.com/free_online_article/sports_body_trai...

http://www.t-nation.com/free_online_article/sports_body_trai...

These two books are essential reading for people with back problems and should be reasonably accessible to anyone with both an athletic background and an advanced high school understanding of anatomy and physiology.

http://www.amazon.com/Low-Back-Disorders-Evidence-based-Reha...

http://www.amazon.com/Ultimate-Fitness-Performance-Stuart-Mc...