In this blog post, Andy Beaulieu enhances the collision detection to handle both Vector graphics and Raster graphics in Silverlight 3.
Looking at the demo code, one of the main routines is the CheckCollisionPoint class, which does a HitTest on a UI Element at a specific Point. If the element is an Image, then a WriteableBitmap object is used to check the specific pixel point (this WriteableBitmap is created in the CheckCollision method, shown later.). If the element is not an image, then we assume it is Vector based and we use the VisualTreeHelper class for a hit test.