This is just so cool! Now with .NET 3.5 Sp1, visual effects running on the GPU can be created and applied to WPF-components. There will be a set of effects provided for you like shadow and blur, or you can create your own with HLSL. The hittesting will still work and if the target computer can't handle shader language 2.0 or higher, it will automatically switch to software rendering.
To apply an effect you only need to write:
<Button ... >
Read more about WPF and GPU Effect here (it is a very good series of post): http://blogs.msdn.com/greg_schechter/archive/2008/05/12/a-series-on-gpu-based-effects-for-wpf.aspx