There is a very talented photographer who posts his works on flickr. He goes by the name of BasicShape. He has photo retouched a couple of images from the past to show how the buildings look today in the same image.
here are the ones he has done so far
2nd street tunnel looking west from Hill Street
Classic Photo Cushman Gallery, new photo and montage by BasicShape
Brunswig Building
Classic photo USC Digital Archives, New photo and montage by BasicShape
Here is a link to his photo stream on flickr, he has very nice Los Angeles photos
http://www.flickr.com/photos/basicshape/