Since Kate will not be arriving in the traditional horse drawn carriage and has chosen a more modern arrival via Rolls-Royce, I believe she has chosen a dress a little less traditional for a royal wedding. I don’t expect her to wear a ballgown or something as grand as Princess Diana’s wedding dress.
I assume she will wear something of more like an a-line silhouette or an empire waist. Since it is a royal occasion taking place inside a cathedral there will be no strapless or sweetheart necklines (no cleavage in front of the Queen, my love). I wouldn’t be surprised if she showed up in a dress similar in style to that of Princess Grace of Monaco.
Kate will wear a dress of understated elegance made of lace and pearls.