According to UStream, they are not in an exclusive agreement with Sony for just the PS4 and states their goal is to provide the same type of service and features to both current and next gen consoles.
Unless Microsoft states the next Xbox can provide the same service on launch day, most likely the PS4 gets the service first while UStream works on being available for other platforms.
http://www.ign.com/articles/2013/02/...1437f210000012