David F. Rico has released the results of a 3-year study evaluating the Effects of Agile Method on Website Quality for Electronic Commerce (ppt or pdf). You can find a reference to this work and others on our Surveys and Studies page.
This new study breaks ground in a number of areas including its indepth research and presentation of related literature and the creation of a new conceptual model to measure agility across multiple agile methods. In summary, he found that 80% demonstrated a correlation between agile methods and website quality. However, you should dive into the report yourself as there is a tremendous amount of valuable information presented.
To contribute to his ongoing research, you can fill out his online survey.