Minecraft wiki entry for Development, associated text: Changes
The world is now 1024x64x64 instead of 512x64x32 in size, and generates actual terrain.
Terrain has been updated to the 0.0.13a_03 format.
Dirt layer is now more uniform.
There is always a single block of dirt after grass and before stone.
If water generates next to lava on world generation, it will now produce stone.
Reference
Minecraft IRC logs, May 21, 2009
"(10:48:56) Notch_: and I just tried making a 1024x64x64 map. It's nuts [...] (10:49:06) Notch_: it's basically a looong gauntlet of hills and lakes"
Trivia
It is very likely this version has the new level generation screen, however this is not confirmed.