I don't expect all fat people to have slow metabolisms, but I expect slow metabolism to be more prevalent among fat people as compared to thin people.

That's not quite the claim I was addressing - the claim is that generally speaking, obesity is the result of having a slow metabolism. But anyway, I was able to dig up some evidence:

First, a video which is obviously not a scientific study but still pretty compelling:

http://www.youtube.com/watch?v=KA9AdlhB18o

In this video, a fat girl who believes she is fat because of a "slow metabolism," is tested. It turns out that she has a perfectly normal metabolism but eats a lot more than she realizes.

Here is a similar video:

http://www.youtube.com/watch?v=WGLwzbvx4S4

Scientific research would seem to indicate that this is pretty typical. i.e. people are fat not because of their "slow metabolism" but because they eat a lot more than they realize:

http://www.nejm.org/doi/pdf/10.1056/NEJM199212313272701

Here is a study which looked at a group of people and determined that the fatter subjects had higher metabolic rates than the thinner ones:

http://ajcn.nutrition.org/content/35/3/566.short

If you search, there are a lot of studies like this out there. AFAIK, most (but not all) scientific studies have found that obese people have higher metabolisms than thin people.

My guess is that this is largely because obese people regularly overeat and so their metabolisms rev up a bit in a futile effort to handle the onslaught of calories. There may be a few studies out there which indicate that obese people have slower metabolisms, but I suspect the differences are pretty minor when you look at the real problem: Fat people eat a lot more than they realize, which is something that every study looking at this issue has concluded.