Jeff Prosise will show you how to use the Navigation Framework to allow forward and backward browsing to work.
A few weeks ago, I blogged about a sample application I wrote that uses Silverlight 3's WriteableBitmap class to draw views of the Mandelbrot set. I hadn't used it long before I realized that it really needed Back-button support. You could drill down into the Mandelbrot set, but you couldn't drill back out. Enabling the browser's Back and Forward buttons seemed a natural way to remedy the situation, but Silverlight and the Back button have never gotten along very well.