I really wanted to do this to see how it would turn out. Basically used a dirt vertex map thingy.
P.S This is done in the 2.7 preview, and their is no option for it under "blender version" so it may or may not be backwards compatible. If it isn't then I'll do a 2.69 version.
To use this shader on your own objects as intended, your objects will need a vertex color "dirt map."