I think no matter how you do yoga, you benefit. Whether you're 100% focused on your practice, or doing poses while you watch American Idol, you're getting something out of it. I've been doing yoga off and on for twenty years (hey now, I started REALLY young!). It's only been recently that I've come to gain some of the benefits of mindfulness. With the help of some good instructors, I've been learning to concentrate on my body while I'm practicing. Or should I say, I'm learning to clear my mind of all thoughts other than the ones about breathing and moving in the direction of the pose. What I notice now when I'm off the mat is that I'm aware right away when my body begins to react to stress. Because I know what it feels like to be completely relaxed, I can feel the small tightening of my gut or clenching of neck muscles before it turns into all out anxiety or pain. Next time you bust out a yoga pose, try to let go of everything else and be fully in that moment. For me a good instructor who has a nice voice is crucial!
I think mindfulness is one of the top benefits of yoga. So often we go through our days absent-minded and not really paying attention to what's going on around us. We also tend to get disconnected from our bodies. Yoga helps us to get grounded and center ourselves in our body.