I picked up some CD labeling pens at some store (compUSA?). They work well, but don't lay down as solid an ink as the good old sharpie. However, they won't eat their way through the substrate, either. Pale would be a good word to describe the ink.

I have been using Sharpies on my printable DVDs. They have a thick white coating designed to be printed on and I figured it was safe to use them on that (famous last words).