DreamWorks Animation has posted the synopsis for Kung Fu Panda 2 to the official KFP site.  The gist is that Po (voiced by Jack Black) and The Furious Five must battle an enemy who plans to use a weapon that can "destroy kung fu"  to conquer China.  Perhaps this device is what the possible subtitle, The Kaboom of Doom, refers to.  As you can see in the image above, DreamWorks has added a "2" to the Kung Fu Panda type to give the sequel a logo, as well.

Jack Black, Angelina Jolie, Seth Rogen, Lucy Liu, David Cross, and Dustin Hoffman are all set to reprise their roles from the first movie; they will be joined by Gary Oldman, Jean Claude Van Damme, Michelle Yeoh, and Victor Garber for Kung Fu Panda 2's May 27, 2011 3D release.  Hit the jump for the full synopsis.

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As seen at the official Kung Fu Panda site:

Po is now living his dream as The Dragon Warior, protecting the Valley of Peace alongside his friends and fellow kung fu masters, The Furious Five - Tigress, Crane, Mantis, Viper and Monkey.  But Po's new life of awesomeness is threatened by the emergence of a formidable villain, who plans to use a secret, unstoppable weapon to conquer China and destroy kung fu.

It is up to Po and The Furious Five to journey across China to face this threat and vanquish it.  But how can Po stop a weapon that can stop kung fu?  He must look to his past and uncover the secrets of his mysterious origins; only then will he be able to unlock the strength he needs to succeed.