One exciting feature added in Silverlight 2 is called ObjectAnimationUsingKeyFrames.  It allows you to target animations at properties that are not Double, Point, or Color.  One of the most basic scenarios is to animate the Visibility property of a UIElement.  Here is an example:

<Storyboard Duration="0:0:2">

  <ObjectAnimationUsingKeyFrames Storyboard.TargetName="myRect" Storyboard.TargetProperty="(UIElement.Visibility)">

    <ObjectAnimationUsingKeyFrames.KeyFrames>

      <DiscreteObjectKeyFrame KeyTime="00:00:01">

        <DiscreteObjectKeyFrame.Value>

          <Visibility>Collapsed</Visibility>

        </DiscreteObjectKeyFrame.Value>

      </DiscreteObjectKeyFrame>

    </ObjectAnimationUsingKeyFrames.KeyFrames>

  </ObjectAnimationUsingKeyFrames>

</Storyboard>

Please note that in Silverlight, the only way to specify an DiscreteObjectKeyFrame's value is using property syntax (using the DOKF.Value tag).  Silverlight 2 does not support WPF's x:Static markup extension.  In WPF, you could specify the value using:

(note: this is NOT supported in Silverlight)
<
DiscreteObjectKeyFrame KeyTime="00:00:01" Value="{x:Static Visibility.Collapsed}">

Have fun with ObjectAnimationUsingKeyFrames!