Using .new as a factory method. Good idea or crazy?

serial_number.rb

class SerialNumber
  def self.new(str = nil)
    # Our new method is really a factory to one of the concrete subclasses.
    if SerialNumber == self
      if str.blank?
        nil
      # Zigbee serial numbers are easy to spot; they start with eight '0's.
      elsif str.strip =~ /^00000000/
        ZigbeeSerialNumber.new(str)
      else
        FtdiSerialNumber.new(str)
      end
    else
      super
    end
  end

  # general implementation
end

class FtdiSerialNumber < SerialNumber
  # overridden implementation
end

class ZigbeeSerialNumber < SerialNumber
  # overridden implementation
end