VisualStateManager, a.k.a. VSM, I quote our WPF architect John Gossman's words from the architectural view:

VisualStateManager, by providing an abstraction of a VisualState, makes unifying the deskop and web models much more straightforward.  While currently in Silverlight a VisualState contains a Storyboard, in the future it contain a group of Setters, just as in WPF.  And while currently the user must call VisualStateManager.GoToState in code, in the future a GoToState action in the body of a Trigger will allow WPF style "codeless" templates to utiltize VisualStates and get the benefits of automatic transition generation and a more designer-friendly tooling experience. (details...) 

Here are a few good resources about VSM:

More to come...