gistfile1.rb

#Models
class Vaga < ActiveRecord::Base

  has_many :idioma_vagas
  has_many :idiomas, :through => :idioma_vagas

  accepts_nested_attributes_for :idiomas, :allow_destroy => true
  accepts_nested_attributes_for :idioma_vagas, :allow_destroy => true

  def idiomas_attributes=(attributes)
      attributes.each{|i|
        idioma = Idioma.find(:id => i[:id].to_i)
        self.idiomas << idioma
      }
  end
end

class Idioma < ActiveRecord::Base

  has_many :idioma_vagas
  has_many :vagas, :through => :idioma_vagas

end

class IdiomaVaga < ActiveRecord::Base

  belongs_to :vaga
  belongs_to :idioma

end

#views
<% form_for(@vaga) do |f| %>
  <%= f.error_messages %>

  <%= f.label :nome %>
  <%= f.text_field :nome %>

  <% prefix = "vaga[idiomas_attributes][]" %>

  <% fields_for prefix, :idiomas, :index => nil do |i| %>
      <p>
        <div>
          <%= i.label :id, 'Idioma:' %>
          <%= i.select :id, Idioma.all.collect {|k| [k.nome, k.id]}, :include_blank => true %>
        </div>
      </p>
  <% end %>

  <p>
    <%= f.submit 'Ok' %>
  </p>
<% end %>