On the blog here, I have this little flickr stream down the page the has goes through all sorts of Lego mosaics on Flickr. One thing that I've seen come through there is this a mosaic of Campbell's Tomato Soup.
Created by TheWorldInBricks (or David Haliski), the mosaic is 30"x40" and is very impressive.
You can check out David's website here. Make sure you do so because he has some pretty sweet creations. In this one section you can interactively walk though a gallery. Incredible!