Bukkit

Eternal Ice worlds can be set as the default world. This means that a new player will spawn in that world.

server.properties

This file is found in the root of your server. Open it with something like Notepad. Find the line that starts with

level-name=

The text after the = sign is the name of the world your server will use. By default it's "world". You should note this name down or change it to something else, for example "world_snow".

bukkit.yml

This file is also found in the root. Add the following code to the bottom of the file (below Database info):

worlds:
  Yourworldnamehere:
    generator: EternalIce

(And, of course, change Yourworldnamehere to the name you used in server.properties.)