See bottom of post for update.
This strapless white dress by D&G Dolce & Gabbana retails for $1375 at StyleBop. Runway photo is from Style.com.
This white lace ruffle Topshop dress is incredibly similar to the D&G version above for a much lower price. It retails for $100 and you can click on the photo below to purchase.
*Update - 4/22/10* - Reader Lauren recently emailed me this option from Bebe which retails for $119. It has it's differences, making it more of an "inspired by" find but it's definitely another option if you're looking for a girlie white dress for summer. Click on the photo below if you'd like to purchase!