Apparently, 14 clinical trials comparing a low carb diets with low fat diets with statistically significant results have been published between 2003 and 2008. In all 14 trials, those on low carb diets lost significantly more weight than those on low fat diets. The author is careful to point out that he only selected studies that statistically compared one diet to another, and that the "low carb" diets were truly low carb. (Some studies claiming to compare low fat with "low carb" are not really low carb, as there are more than 30% of calories from carbohydrates. In a true low carb diet, carbs are usually 5 to 10% of calories).
The author of the article summarizing this list has given permission to copy it onto other blogs, so I am taking advantage of his generosity. Here are the studies, complete with links, for you data junkies out there.
Randomized controlled trials showing significantly more weight loss with low carb diets: