After eight years in prosecution, US patent 7,298,851 is now in play:

Discovery Communications, parent company of the Discovery Channel and Animal Planet, has filed a complaint against Amazon.com alleging that some security and copy protection features in the Kindle and Kindle 2 violate the company's patents.
According to a copy of the suit, Discovery charges that Amazon violated its patent for Electronic Book Security and Copyright Protection System. The patent, U.S. 7,298,851, was issued to Discovery Communications by the U.S. Patent and Trademark Office on Nov. 20, 2007, per the lawsuit filing (PDF). It was initially filed in 1999.