Have you ever used Photoshop (or similar) to give a button or panel a 3D appearance using inner-bevels etc?
Phil Middlemis describes an easy way to achieve the same thing but with 100% XAML and completely scalable.
Some of the graphics were understandably left as raster since there would be no equivalent way to achieve the same result in Blend. But the elements that bothered me were usually simple panels or buttons [...] had rounded corners, and they had some kind of bevel that made them appear raised or lowered.