I got the statistics from Tipping the Scales of Justice: Ending Weight-Based Discrimination by Sondra Solovay, which includes citations of the relevant studies. I don't have my copy to hand at the moment, as I'm at work (and won't be home till Sunday evening).

The studies on long-term dieting success define it as "maintained weight loss for two years", mainly because it has been extremely difficult to find sufficient people who have maintained it for longer. Solovay has some interesting comments on how difficult it was even to recruit people who fitted the definition above, and why even the 10% figure may be optimistic.