Here is an interesting article on the dangers of yoga. While many people perceive Yoga as being very gentle and relaxing, it can have its risks. Like any physical activity, you need to know what you are doing. Everyday, I see people at the gym with horrible technique, and a few injuries every now and then. Yoga, like strength training, you need to know what you are doing. If you don't, you need to hire a competent teacher. Unfortunately, there are a lot of incompetent teachers (or just regular exercisers who don't have any formal training). Many commercial gyms offer yoga classes, but have inexperienced instructors with little formal training in anatomy, biomechanics, and exercise physiology. Also, the classes are large, they sometimes mix beginners, intermediates, and advanced yogis. Students also need to take some reponsiblity, too. Many times the student is looking for a good "workout," is inconsistent, and feels pressure to keep up with others in the class.
As far as the yoga discipline, I have mixed feelings. You will increase flexibility, strength and relax with yoga. However, some joints don't need to have increased flexibility, such as the neck and lumbar spine, and some people's shoulders. Flexing or hyperextending the spine increases forces on the ligaments and discs. The lumbar spine (lower back), for example has a very small range of motion (<10 degrees). I wouldn't recommend some poses (ie. the Plow, heard of many traumas from this one- herniated discs, fractured vertebrae), however, I use some, regularly (ie. downward dog). The bottom line is if you are going to pursue any discipline (Yoga, Weightlifiting, kettlebell lifting, etc.) find a qualified teacher, have patients, be consistent, and pay attention to your body.