Hi Jon,
If I were you , I wouldn't put it right on the original file. This could easily be done in post production (lightroom, photoshop or Aperture). Because if ever you want to have a clean pic, you'll have it then.
I do this when I do export my work copies from Aperture.
Sorry, but I think I misread the original post. I thought you were talking about the filigram signature on the actual picture. Pavement pilot is right, there is an option for copyright, but I don't know where it goes from there.
Marcel