answer on deleted comment about Box2D camera resizing

http://www.youtube.com/watch?v=dF5dUiJtYng&lc=ho4m9VeFjkmwbELgFTVA_xDJMZ7JUu2IyYwHbl__KtQ&lch=email_reply&feature=em-comment_reply_received

Just put this in the resize method, it should do all that's necessary:
camera.viewportWidth = width / unitScale;
camera.viewportHeight = height / unitScale;
camera.update();
That's actually what's in the code of this tutorial as well, check out the source in the description.

If you set the projection matrix of the SpriteBatch to the combined one of the camera (like you usually do every frame), it should work without problems:
batch.setProjectionMatrix(camera.combined);