Take a look at this article posted at the Expression Blend and Design blog in which Kirupa Chinnathambi talks about the ControlStoryboardAction and the StoryboardCompletedTrigger.
Storyboards are one of the primary ways you create animations in Silverlight, WPF, and Windows Phone using Expression Blend. Creating a storyboard is fairly easy, but actually using a storyboard such as having it play is not. To help with this, you have the ControlStoryboardAction.