This topic was JUST covered in The Paleo Solution Podcast last week- episode 55: http://robbwolf.com/2010/11/23/the-paleo-solution-episode-55/

Yes, I've come across this in a bioethics context. Given that prader-willi is such an extreme genetic disorder with so many different symptoms attached to it, it's not likely that dietary adjustments would have a massive impact on it. Obviously it might be possible to intervene to mediate some of the symptoms (in the same way that if they were limited to only eating lettuce all day, presumably it would stop them becoming morbidly overweight), but the various factors that cause them to be insatiably hungry (disordered hypothalmum, hormones etc) seem to be relatively innate, rather than environmental. Note of course that even if dietary adjustment did remove the obesity, it would still leave the mass of symptoms asociated with PWS untouched.

In the 1980s when I was in my 20s, I lived at NIH for a month to participate in an obesity research protocol. Also in that study were a number of children with Prader-Willi Syndrome. At that time, they were having success using a 500 calorie/day, almost no carb diet so that the subjects were in a state of ketosis, which seemed to help the whole uncontrollable appetite problem. I don't know what the recommendations were for a long term approach using this method. I have thought recently, after following the Paleo Diet for a year, that it would be very interesting to see if Paleo would help people with this disorder.