In this blog post David Anson explains how to animate the Horizontal/VerticalOffset properties of a Silverlight/WPF ScrollViewer control.
Just in case you've never tried this yourself, I'll tell you that it's not quite as easy as you'd think; those two properties are both read-only and therefore can't be animated by a DoubleAnimation. The official way to accomplish this task is to call the ScrollToHorizontalOffset or ScrollToVerticalOffset method - but of course that can't be done by a DoubleAnimation either.