Yes, it can. Faces is a server-based framework that supports different render kits, whose goal is to represent components in a way the clients understand. The most common client is an HTML-based web browser, but as long as the client understands how the data is being sent, it could be any range of client systems, including custom applications and applets. At least one company, Droplets, which specializes in rich client user interfaces integrated with servlets and EJB, will be supporting JSF.