announced today a new map format for Minecraft
to prepare for modding, especially to allow for more block types, but also to improve other areas of the game (such as map height). Maximum build height has been increased to 256 (was 128), empty sections of the world are not loaded into memory, and block ids have been increased to 4096 (was 256) by adding a 4 bit data layer (similar to how meta data is stored). In addition, block ordering have been changed from x/z/y to y/z/x in order to improve compression, and packets for sending chunks have been updated (a full 128-high chunk is smaller than the old format, and a chunk with lots of empty space is much smaller). The files of the current format (“McRegion”) will remain in your save folder, if you need to restore them for some reason. Visit this page
for more details.