In Aperture 2 it is really simple.
Make a copyright template .psd file with transparent background in Photoshop or Elements, then whenever you export a file out of Aperture (this opens up an options window) there is a box to check if you want to add a watermark to the exports as you choose export type, size, and quality.
You can even adjust transparency of the watermark and have Aperture scale it to the image. I do this to hundreds of files at once as I make copies of originals resized for web sharing. Piece of cake...
In the following image, all I did was tell Aperture to scale watermark and use 30% transparency, the photo is from a batch of 100 all done at once!