Omnivision's recent patent application US20110068429 talks about providing the contact to the backside of BSI pixel array. Ideally, these contacts could be placed inside each pixel in the array. For small pixels this requires hard trade-offs in the pixel layout. Hence Omnivision proposes to sacrifice a small number of pixels for dummies having backside contacts like this:
The application talks about devoting one pixel in thousand to such a backside contact (one in each 32 x 32 block, or, may be, pseudo-random). The application says that "already established and applied methods of averaging out white or dead pixels in any CIS device will also remove the impact of the contact-dummy pixels from the final image. Therefore the final image will not significantly be altered by contact dummy pixels."