Pocketgamer took note of a Finnish site that posted images of the expansion which, if it follows the pricing of the previous holiday special, Angry Birds Halloween, should come crashing in at $.99. There has not been an official announcement made yet.
Developer Rovio has dubbed December 11 Angry Birds Day and has set up a website for happy players to meet up all around the world and form flash mobs. Further details regarding your mob's activities will arrive by email closer to the date -- don't worry, it will allegedly be suitable for all ages. Then again, it's probably wise to be cautious when the basis for a meet-up is bird flinging.