I replaced a pic of our son with a blank piece of paper (well, DS scribbled on it) that said "sibling pic- coming April 2013". I did two others similar to that. DH didnt see them, but I knew when DH asked DS what he did all day he would pointed them out.
He still didnt "get it" right away and to be honest was annoyed that I didnt just tell him. He even made me promise not to do anything fun for the gender reveal (he will be out of country). Ah well. My mom appreciated the fun!