David Anson shows you how to easily animate LayoutTransformer with AnimationMediator.
I wanted something that was simple and easy to use - and managed to come up with a pretty reasonable XAML-only solution on my second try. [The first attempt would have been a bit simpler, but didn't work out. :( ] The basic idea is to insert a Mediator between the animation and the LayoutTransformer and let the Mediator make sure the LayoutTransformer is updated when necessary. This task is complicated slightly by the fact that Silverlight won't let you create a Binding on one of its Transforms - but the equivalent effect can be had by putting a TwoWay Binding on the Mediator instead (as discussed in more detail by Jeff Prosise here).