http://www.mojang.com/2012/02/…necraft-map-format-anvil/
4096 block IDs (putting modders first!), 256 height (sea level still 64), biome data stored in the map, smaller SMP chunk packets, empty chunks no longer loaded in memory and other internal changes.
Will ship in 12w07a and 1.2 - yes, you'll be able to convert your old mcregion worlds to the new format.
Modders rejoice!