http://reviews.photographyreview.com/blog/canon-eos-50d-announcement/
taken from the a/m site
Canon has introduced new gapless microlenses for each pixel and a new four-step user-selectable noise reduction system.
Canon EOS 50D Image Quality
Everyone should know by now that increasing the number of pixels doesnt automatically mean better image quality. On a basic level, adding more pixels to the same size sensor means smaller pixels and more noise. To accommodate the 50% increase in pixels without compromising image quality, Canon has introduced new gapless microlenses for each pixel and a new four-step user-selectable noise reduction system. Over the past two years, improved noise reduction and increased light-sensitive area on individual pixels has meant improved image quality even while pixel counts have increased. The gapless microlenses sounds like another strategy for getting more direct light to the pixel. And 50% more pixels means noise will be effectively minimized in prints or Web images. The ISO 6400 and ISO 12800 high sensitivity settings are exciting but without seeing full-size samples theyre impossible to judge. However, its not likely that Canon would include them if they couldnt compete with Nikons high-sensitivity cameras. So well just have to wait and see if the image quality measures up. But over the past couple of years Ive been surprised at how image quality has improved even while pixel counts have gone up. So Im prepared to be impressed