I first saw this on 2 Craft and 2 Create and thought it was a good idea, and so put the thought aside for a time when we wanted to create a birthday present for a grandparent.
At first I bought a bunch of multi-coloured permanent marker sharpies. The kids drew up some beautiful pictures. I cooked them, and then easily wiped them off with a wet cloth. Not happy. Moral of the story:
Don't use colours!
Second attempt in black and white and some even better pictures, a little baking (20 minutes on 180 C or 350 F), and we had our finished products. Personalised coffee cups, fit for any Mothers day, Fathers day or Grandparent's birthday.