I get this question from time to time and my short answer is "maybe." Here's a longer one I sent to a student today:
"Yoga can help with weight loss, but doesn't necessarily. I, for instance, am no skinny minnie :) I had a couple of surgeries in June 2011 and gained substantially year I was effected and "grounded". I'm loosing gradually over time, but not precipitously. While this is the absolutely most healthy way to loose weight, it is not the way most folks choose to loose weight.
The way you eat will have a great effect as well. If you previously ate lots of refined carbs and less health oriented foods and cut them out, that will have a great effect rapidly. Yoga also helps turn your appetites toward fruits and veg over time, which is a great help. If you've been eating pretty health consciously all along, just maybe too much, you won't see as immediate effect - but the effects do build up over time.
For the greatest effect on weight loss, plan to practice in class or on your own about an hour a day 5 or 6 days a week. 4 of those days you'll want to engage a pretty yang (strenuous) practice, perhaps vinyasa (which I favor for strong practices). 1-2 days a week you'll want a deep restorative practice, which I also teach. At least one day a week should be an "off" day - walking, hiking, swimming if you'd like - but not yoga. You might choose to have an oil bath on your off day (we can talk about that as you're ready).
Sleep is foundational for weight loss and yoga will encourage sleeping well. These practices will also calm your nervous system, encourage parasympathetic tone (the feed and breed side of the autonomic nervous system that balances the fight or flight) and so help with any anxiety or other issues.
All in all yoga is a pretty solid way to get in touch with your body and this is the most effective weight loss technique there is. Yoga will also help you befriend your body so maybe your feelings about your weight will become more friendly, which is also a big help.