This how-to will show you a few simple steps to making a clipping path in Blend 4.  Below is a mockup of my basic canvas (this is a PNG imported into Blend).  I want to make the green area visible and I want to clip the while rectangles out of the image.  Eventually, I will place some animated elements where the white rectangles are.

http://buildsucceeded.com/oldpics/game-mockup

The first thing you want to do is to draw rectangles where you want the image to be visible.

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Now that I have all of my purple rectangles drawn out I am ready to convert them to a clipping path.

Remember, the rectangles are the areas that will be visible on the clipped object.

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Choose the direct select tool: image and select all of your rectangles.  Hold down the CTRL key and you can multi-select.  You can also select the rectangles from the Objects and Timeline window.

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Choose Object | Path | Convert to Path from the Blend menu.  This converts all your rectangles to a path which we will in turn convert to a clipping path.

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Next, go back to Object | Path on the menu and choose Make Compound Path.  This will make one complex path from your individual paths.

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Now that you have one compound path select the Path object from the Objects and Timelines window.  Now you can go back to Object | Path and you have the option to Make Clipping Path.

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Now you choose the object to clip and you will now have a clipped object (an image in my case). 

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Here is the end result in the Blend designer:

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And here is the nice clean XAML you generated (please note, for space constraints I truncated the path data in the Clip property):