This is a classic technique to give your renders a wireframe overlay on top of your existing materials. It's been in Blender for some time, though it seems to slip through the cracks every so often. But never fear, BlenderNewbies has a video tutorial on how to do it!
Update [bart]: new script version
You can find it here. And if the process is a bit slow for you, Francois Gueguen (Babynou), has created a neat script that does the work of creating a wireframe overlay on top of an object. The real power of this script is that you can add a wireframe material to all objects, selected objects, or just the objects on the active layer. You can find the script here (direct link to the script). Since everything is in French I did an English translation of the script which you can find here (direct link to the script).
Update [bart]: there's a new version of this script out which is faster and allows you to control the subsurf level.