I like this: Good quality porcelain mug with four art pens. Designs can be re-done over and over or bake them to make them permanent. Mug Design.
This makes me want to buy one of these sets and send them to some of my favorite illustrators (such as Camilla Engman) and kindly ask them to create a spontaneous-custom-surprise-single-edition-mug for me. Oh, the possibilities.. this could turn into a frantic-world-wide-mug-design-trade.