You'll need the 12w21b client to play on NueCraft.

Here's how you can get it all set up.

1. Go to http://assets.minecraft.net/12w21b/minecraft.jar and download 12w21b's minecraft.jar file.

Windows XP Users;

Click Start, then click Run
Type %appdata% into the Run window that appears
Click on .minecraft
Open the bin folder
Replace the existing minecraft.jar with the 12w21b minecraft.jar

Launch/Restart Minecraft!

Windows Vista/7 Users;

Click Start, then type %appdata% in the search module
Click on .minecraft
Open the bin folder
Replace the existing minecraft.jar with the 12w21b minecraft.jar

Launch/Restart Minecraft!

Mac Users;

Use Windows as your operating system - Mac is too nooby.

NOTE: You won't be able to play on 1.2.5 servers (or anything below it)

Additional Notes

A small server that I've decided to make. TNT is enabled, so are Creeper Explosionsssssss... Play on Survival, have some fun..