The most obvious physical benefits of yoga practice include loosening of muscles that have been tightened by inactivity, tension, and stress. Asana practice also increases the range of motion of joints, enhances flexibility, and can help correct postural problems that may have resulted from weight gain. Any style of yoga helps tone, lengthen, and strengthen the muscles, which can contribute to the sculpting of the body, but not necessarily to weight loss. Remember that muscle is, after all, denser and therefore heavier than an equivalent volume of fat tissue. According to yogic tradition, asana practice also gets the prana of the body moving, which can be helpful for you if weight gain, decreased energy, and sluggishness have appeared together.
Yoga also offers psychological benefits. Weight gain often brings with it a great deal of harsh self-judgment. Through yoga, we can counteract this by creating a safe, positive environment to reconnect with our bodies and quiet the counterproductive messages that often arise in our minds. Reengaging in physical activity through asana practice can also foster a renewed sense of control over our lives, a quality that sometimes diminishes as one's weight refuses to budge!
On a physiological level, certain styles of yoga could be more appropriate for students who have weight loss as a primary intention. Vinyasa-style class, where movement and breath link poses together, can build heat and potentially result in greater calorie burn. This style of practice could supplement other aerobic exercise that you're involved in, such as walking, running, biking, or swimming. Take it slowly, though. Something as intense as the Primary Ashtanga series may not be the place to start if you haven't been physically active for a while. Begin with a good introductory vinyasa class.
Moreover, there are more and more yoga resources promoting the practice for weight loss. In the September/December 2002 'Yoga Research and Educational Centernewsletter', editor and yoga instructor Richard Rosen reviews a recent book, Yoga Burns Fat, by Jan Maddern. He states, "The premise of the present book is that yoga practice has two prime benefits for people wanting to lose weight: one, it improves digestion and so eliminates constipation, water retention, and bloating; and two, it improves blood circulation to major endocrine glands (such as the thyroid and pancreas) that 'control your appetite, moods and sleep patterns'... as well as improved self-image." Rosen warns that, as is often the case, these premises are not supported by any scientific research, and he notes that the photos in the book, in his opinion, could be better. So, will hatha yoga practice help you lose weight? Maybe. Will it change your relationship with your body? Most likely, and probably for the better.