I just came across this fascinating agency in Berlin: http://www.schein-berlin.de
These guys create all sorts of fictional artefacts: e.g. packaging of products that never really existed, newspapers for film productions, fictional stamps and labels, everything. Amongst many other productions and projects I found out they had been working e.g. for "Bourne Supremacy". I like their work because it is such a special synthesis of Art and Graphic Design. Highly recommended. Unfortunately their website is only in German (yet).