When Mojang releases an update to Minecraft, there is a delay in updating the servers. Before we can update, we need the Bukkit team and plugin authors to update for the PvP and Build servers.
Preventing An Update
The latest Minecraft launcher, available for download here gives you an option to prevent an update each time you load the game. It is STRONGLY recommended you download this before ever playing Minecraft again.
If you've already updated, someone may have provided a minecraft.jar file which you can use to overwrite the newest version you've downloaded.
Place the minecraft.jar file in the %appdata%/.minecraft/bin folder, overwriting what's currently there.
Place the minecraft.jar file in the /Users/[username]/Library/Application Support/minecraft/bin folder, overwriting what's currently there.