If you aren't already aware, there is an interesting legal battle going on at Wrigley Field. The Cubs ownership wants to install a video board, but this board would block the view from the neighboring rooftops. The interesting economic question to me involves both externalities. As it is right now, the Cubs generate a positive externality in the form of games being viewable from neighboring rooftops. Putting a large video board along the outfield wall would block this view to some extent.
Paul Sullivan has an article in the current Chicago Tribune that gives some information on what's going on currently.