With Silverlight 4, a new StateGroup called LayoutStates has been added to the ListBoxItem class. Expression Blend 4 exposes LayoutStates, and allows you to manipulate and add different effects and animation when ListBoxItems are added or removed from the ListBox control. This recipe gives you some ideas and a simple approach on how to start using those effects and get you up and running with LayoutStates.
Start your Expression Blend 4 and create a new project. Make it a Silverlight 4 project and from the Language drop-down list choose C#.