Hanafuda cards are a special kind of Japanese playing cards, originally imported by the Portugese three centuries ago, and became a Japanese cultural icon. The company Nin Ten Doh was originally founded as a maker of hanafuda cards, for example. This official card set is based on 2001's The Spiriting Away of Sen and Chihiro (yes, that's the full translation of the Japanese title), and fits the characters perfectly into the classic designs.
The only reason I know anything about hanafuda (aside from Nintendo's brilliant Clubhouse Games on Nintendo DS) is because they feature prominently in two of Isao Takahata's movies, Jarinko Chie and My Neighbors the Yamadas. If you have a keen eye, you can probably spot some of the cards that were used in those films. For that reason, the idea of Spirited Away hanafuda cards makes perfect sense.
This is the sort of thing you expect to find at fan conventions. Collectors and die-hard fans go gaga for memorabilia of this sort. Definitely one of Studio Ghibli's more offbeat merchandising ideas.