announced today that a new map format have been added for Minecraft
allowing 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). Visit this page
for more details.