In this Pre-Release 6 mojang patches the following that will be available when 

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

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1. Added a defense mob to villages

2. Added up-side-down stairs

3. Many changes to the world and lighting system




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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.