I may be really off, but on the long shot that I am right, I want it to be published for all to see!
I think Catherine will be in a slim fitting lace gown similar to any of the Elie Saab dresses above! She has such an amazing slim and tall frame that this style gown will look incredible on her! The lace gives the gown such an elegance and femininity I think Kate will be drawn to. Mila Kunis dress from the 2011 Oscar's has an stunning shape, but may be too revealing for the Cathedral setting. A soft cap sleeve and more conservative neckline like the dress below will be more Kate's style!
I don't think that Kate (Catherine) will be wearing a skirt to this volume, but I do think that the regal fabric and lace cap sleeves are very likely to be seen on Catherine on Wedding day!
I'd love to hear what you think the Princess in waiting will wear on her big day! Speak now or forever hold your peace!