Apologies if this has already been discussed, but Hello! magazine has an excellent article about the exhibition of the wedding dress etc and there is a short film at teh exhibition showing designer, Sarah Burton, explaining how the dress was created. To summarize:
With regard to the Veil, Catherine wanted a very romantic effect, but did not want to detract from the bottom of the dress. The veil echoed the arches at the back of the dress.
The skirt being made from silk gazar meant it was lighter and did not feel over-powering or heavy. Inside the dress, bustles were created out of circles of organza (like and oyster effect) which is an old Victoriam technique. Catherine did not want the dress to "collapse" or crumple while she walked up the aisle and wanted to maintain the symmetrical shape without using too many underskirts. Weighted canvas was therefore used at the bottom of the skirt and double canvassing in the middle of the skirt so that when the skirt was lifted it would fall back exactly in the same position. The train also used weighted panels.
Overall, the effect is that the dress has the essence of the Victorian, whilst using light fabrics and pleating to give it a modern feel.