There is a logical distinction between an organized public prayer (I can't think of a better term right now) at a public, state funded school or college and an organized public prayer at a professional sporting event. It's an understandable...note I did not say right or wrong...argument that the former is a governmental act promoting one religion over others, whereas the latter is a public expression of religion by individuals. Whether the Consitutional seperation between Church and State is correctly interpreted is a seperate question. I don't think the fact that a stadium is publically owned is enough to forbid public prayer either, since there is a rental or lease or something like that making its use for a sporting event a private activity. Just my opinion.
So far as I know, all the USSC pronouncements on prayer in stadiums have had to do with schools.