Jens of Mojang has anounced that they are working on a new map format they have code-named “Anvil”. The purpose of this format, as far as I can tell, is to provide a new platform for modding. Perhapse this is in preperation for the modding API they have been working on for a long time?
More details can be found on the original post on Mojang.com:
Jon and I have been working on a new map format for Minecraft that we’ve dubbed “Anvil” (it’s just a name). The purpose of the new format is to prepare for modding, especially to allow for more block types, but also to improve other areas of the game (such as map height).