Well, no transparent letters on the 12" PB since no backlit keyboard, but I do suspect they are plastic.They are plastic (on the MBP at least)... They're transparent plastic painted with silver paint, leaving the letter unpainted so that light can shine through.
Anyhow, I'm going to order some iKlear (or Klear Screen - seems like the same stuff anyway, but different packaging!) and see how it works.