In this article, Mahesh Sabnis has explained how to define custom command behavior to the ListBox control using Silverlight and Prism 4.
Source: Dot Net Curry
Prism 4 is an excellent pattern provided to us for developing Line-of-Business (LOB) enterprise applications using Silverlight 4.0 as well as WPF 4.0. Implementing XAML only development is an excellent feature where the code-behind can be completely reduced or removed. In a typical MVVM architecture, to process the data which is send by the UI (by the ViewModel), we have been provided with an excellent Commanding concept. Command is an object which implements an ICommand interface.