This script will sync your current followings to one of your lists. It will add users to the list who you are following, and remove users from the list who you are not following.

sync-list.rb

require "rubygems"
require "twitter"
require "date"

# get these from apps.twitter.com
CONSUMER_KEY = "foo"
CONSUMER_SECRET = "bar"
OAUTH_TOKEN = "blee"
OAUTH_TOKEN_SECRET = "baz"

TWITTER_USER = "foobar" # needs to be the username associated with keys above
LIST_ID = 1234567890 # this will be a long numeric ID; get it from the list URL

TIMEOUT = 1 # a good default value

client = Twitter::REST::Client.new do |config|
  config.consumer_key        = CONSUMER_KEY
  config.consumer_secret     = CONSUMER_SECRET
  config.access_token        = OAUTH_TOKEN
  config.access_token_secret = OAUTH_TOKEN_SECRET
end

following_ids = client.friend_ids(TWITTER_USER).to_a

list_members = client.list_members(LIST_ID).to_a

list_members_ids = list_members.map do |member| member.id end

following_ids.each do |id|
  unless list_members_ids.include?(id)
    "Id #{id} not in list, adding"
    client.add_list_member(LIST_ID, id)
  end
end

list_members_ids.each do |list_member_id|
  begin
    unless following_ids.include?(list_member_id)
      puts "No longer following #{list_member_id}, so, removing from list"
      client.remove_list_member(LIST_ID, list_member_id)
    end
  rescue Twitter::Error::TooManyRequests => error
    puts "Got an error, probably rate-limiting... waiting #{error.rate_limit.reset_in} seconds to try again"
    sleep(error.rate_limit.reset_in)
    retry
  end
end

puts "Done!"