I was recently visiting Corey Schuman's blog since he has been focusing on Silverlight + Blend development for a while now. Among his posts I found his Blend section which contains information about behaviors and how to include a recommended Physics Library for Silverlight that allows you to create Physics behaviors by using mostly Blend.
I actually went ahead and watched the "Basics" video and I got so excited about how easy it seemed to play with it that I downloaded and installed the Physics Helper library and started to play with it right away. I can't tell how fun and easy is to start playing with it and actually create something. I'm sure that in order to build useful and more complex stuff you will need to dig deeper and get into the C# code, but I believe the library helps a lot and gives to many tools to start with.
In the CodePlex site for the Physics Helper library you will find information, installation instructions and a few videos that will help you to start playing and building things right away.
Thanks to Corey for the information and I hope you jump right away into Silverlight Physics behaviors after reading this.