[JamesBarney]

http://m.ajcn.nutrition.org/content/82/1/222S.full

Here's one that mentions 20%. But that's at a year. The % changes depending on how long you follow up(drops all the way out to 5 years.). This is study uses keeping off 10% of body weight as the definition of success which is far less than the amount that it takes to move an individual from obese into the normal healthy range.

If you can find a study where a majority of the individuals experienced significant and long term(3 yrs+) weight loss I would be happy to see it.