Book Wholesalers, Inc., a book supplier to U.S. public libraries, has posted a small report on their recently received shipment of Harry Potter and the Half-Blood Prince including a tiny picture of the backside jacket.
If you're trying to stay spoiler-free, you might want to pass on this one although it's difficult to see any detail.
UPDATE: Scholastic has officially released a much higher quality version of the art on the back side (without text). From their press release:
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