Extracting frustum planes

[kitfox]

I'm having trouble extracting the frustum planes from a combined modelView and projection matrix.  While the below code seems to work for perspective projections centered on the origin, when I shift the view to the right by 10 units, all of a sudden my planes are all skewed to the right.  Shouldn't they have the same normals as before, and just have new offsets from the origin?

package com.kitfox;

import javax.vecmath.*;

import com.xith3d.scenegraph.*;
import com.xith3d.spatial.bounds.*;
/**
 *
 * @author kitfox
 */
public class ExtractPlanes
{
   
    /** Creates a new instance of ExtractPlanes */
    public ExtractPlanes()
    {
    }

    public static void main(String[] args)
    {
        Transform3D projectionMtx = new Transform3D();
        Transform3D modelView = new Transform3D();
        Matrix4f modelViewProj = new Matrix4f();
        Matrix4f identity = new Matrix4f();
        identity.setIdentity();
        Frustum frustum = new Frustum();
       
       
//        xform.ortho(0, 4, 0, 4, 0, 4);
       
        //Perspective frustum centered on origin
        projectionMtx.perspective((float)Math.toRadians(90), 1, 1, 1000);
       
        modelView.setIdentity();

        modelViewProj.set(modelView.getMatrix4f());
        modelViewProj.mul(projectionMtx.getMatrix4f());
       
        System.err.println("" + modelViewProj);
     
        frustum.extract(modelViewProj, identity);
       
        System.err.println("" + frustum);

       
        //Perspective frustum shifted 10 units to the left
        projectionMtx.perspective((float)Math.toRadians(90), 1, 1, 1000);
       
        modelView.setIdentity();
        modelView.setTranslation(new Vector3f(10, 0, 0));

        modelViewProj.set(modelView.getMatrix4f());
        modelViewProj.mul(projectionMtx.getMatrix4f());
       
        System.err.println("" + modelViewProj);
     
        frustum.extract(modelViewProj, identity);
       
        System.err.println("" + frustum);

    }
}