Build errors about Microsoft.Expression.Interactions?

[ScottBeeson]

The referenced component 'Microsoft.Expression.Interactions' could not be found.

The tag 'ControlStoryboardAction' does not exist in XML namespace 'clr-namespace:Microsoft.Expression.Interactivity.Media;assembly=Microsoft.Expression.Interactions'.

Could not resolve this reference. Could not locate the assembly "Microsoft.Expression.Interactions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.

Is there a trustworthy build somewhere?