I recently discovered the work of Erik Johansson who shares a lot of my beliefs about photography, then yesterday I happened to stumble upon a TED video he did last November. It's only 6 minutes long and well worth the watch, as not only does he talk about how he started creating images like this, but he also shows some of the source images that make up his work
More recently he has worked on an interesting project at Sergels torg in Stockholm where he created a giant trompe l'oeil in the center of the square and is currently working on a project with Google. More details on Erik's Stockholm project can be found in his blog.