In this Pre-Release 5 mojang patches the following that will be available when 

Minecraft 1.2 is Released THIS IS JUST THE PRE-RELEASE.



1. Added new world file format called “Anvil” (256 max height and 4096 block IDs*)
 
2. Multi-player light calculations do no longer cause affected blocks to be 

   transmitted to the clients, instead the clients will recalculate the light on            

   their own.

3. Villagers will repopulate villages based on how many houses there are available.

4. Some nights in villages will be worse than others…

5. Added a redstone-controlled light source.

6. Decreased chance for the rare mob drops.

7. All animals use the new AI system now.

8. Crash bug corrupting levels when spawning mobs above the 255th block
Sheep animation is correct in SMP.

* The game can’t use the full range of IDs yet because there are still some assumptions that IDs above 256 are items.

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INSTALL INTRUCTIONS 


Windows- 

1. Type %appdata% in the search bar 

2. .minecraft/bin

3. change your current .jar file to whatever you like ( Example: minecraft1.1.jar )

4. now just change the current .jar file that you downloaded to minecraft.jar

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MAC - 

1. goto Finder/User/Library/Application Support/minecraft 

2. FOLLOW STEPS 2-4 above.