One of the requests that I hear quite often for Silverlight is support for Alpha Video where video has transparent regions to ease compositing. By adding GPU-Accelerated Pixel Shaders to Silverlight 3, that capability will now be possible.
One of the foundational new features in Silverlight 3 (to be released later this year) is the software-based GPU-based Pixel Shader support like WPF already has. You can use any pixel shaders created for WPF for Silverlight applications. The WPF Pixel Shader Library on CodePlex now has a Silverlight 3 library as part of the source code. One of the shaders in that library is a simple color key pixel shader. I started with that shader and adapted it by adding color and tolerance properties as well as design-time support for Expression Blend 3 and an installer that adds the shader to Expression Blend Asset panel).
You can download the shader installer and source code here.
5/15/2009 – Correction: pixel shaders in WPF are GPU accelerated, in Silverlight 3, they will be software-based.