New in the September CTP is the ScreenSpaceLines3D class in System.Windows.Media.Media3D.  This class is functionally a line primitive.  Syntax-wise, it is quite easy to use. Here is an example:

<ScreenSpaceLines3D
                    Color="Black"
                    Thickness="4.0"
                    Points="0, .1, 0, .1, 0, .1"
/>

The color and thickness attributes are obvious.  The points attribute, in XAML, specifies a collection of Point3D objects.  In the above example, the first point is at 0, .1, 0 and the second is at .1, 0, .1.  You can have more than just two points in this collection, which is handy.

I like this class for a couple reasons.  First, it is a primitive within the framework that you can use to start playing with 3D.  This means you don't have to import any geometry at all or use any code behind. Second, the class is very easy to use and, if you are new to 3D, it provides a very simple way to start playing with 3D, wrapping your head around the notion of a 3rd dimension and a Z axis. Using this primitive, you can play around in XAMLPad and create some very simply geometries and get the hang of 3D space.  For example, I threw 3dpyramid (XAML posted below) together just to give a sense of this, adding a red line to represent the X axis, a green line for Y and a blue line for Z (building on the SDK example).   

<Grid xmlns="http://schemas.microsoft.com/winfx/avalon/2005" xmlns:x="http://schemas.microsoft.com/winfx/xaml/2005" >
<Viewport3D Name="myViewport3D">
  <Viewport3D.Camera>
                <PerspectiveCamera FarPlaneDistance="50" LookAtPoint="0,0,0" Up="0,1,0" NearPlaneDistance=".1" Position=".5,.5,1" FieldOfView="45" />
            </Viewport3D.Camera>
    <Viewport3D.Models>
        <Model3DGroup>
            <Model3DGroup.Children>
                <!-- Add a red screen space line to represent the x axis. -->
                <ScreenSpaceLines3D
                    Color="Red"
                    Thickness="2.0"
                    Points="20.0, 0.0, 0.0, -20.0, 0.0, 0.0"
                    />
                <!-- Add a green screen space line to represent the y axis. -->
                <ScreenSpaceLines3D
                    Color="Green"
                    Thickness="2.0"
                    Points="0.0, 20.0, 0.0, 0.0, -20.0, 0.0"
                    />
                <!-- Add a blue screen space line to represent the z axis. -->
                <ScreenSpaceLines3D
                    Color="Blue"
                    Thickness="2.0"
                    Points="0.0, 0.0, 20.0, 0.0, 0.0, -20.0"
                    />
  <!--Add lines to build pyramid-->
                <ScreenSpaceLines3D
                    Color="Black"
                    Thickness="4.0"
                    Points="0, 0.1, 0.0, .1, 0.0, .1   "
                    />
                <ScreenSpaceLines3D
                    Color="Black"
                    Thickness="4.0"
                    Points="0, 0.1, 0.0, -.1, 0.0, -.1"
                    />
                <ScreenSpaceLines3D
                    Color="Black"
                    Thickness="4.0"
                    Points="0, 0.1, 0.0, .1, 0.0, -.1"
                    />
                <ScreenSpaceLines3D
                    Color="Black"
                    Thickness="4.0"
                    Points="0, 0.1, 0.0, -.1, 0.0, .1"
                    />
                <ScreenSpaceLines3D
                    Color="Black"
                    Thickness="4.0"
                    Points=".1, 0.0, .1, 
                            -.1, 0.0, .1,
                            -.1, 0.0, -.1,
                            .1, 0.0, -.1,
                            .1, 0.0, .1

                     "
                    />
            </Model3DGroup.Children>
        </Model3DGroup>
    </Viewport3D.Models>
</Viewport3D>
</Grid>