viddl.rb

require "viddl-rb"

ViddlRb.io = $stdout

Shoes.app :title => 'Viddl', :width => 300, :height => 150, :margin => 10, :resizable => false do

  stack do
    para "Enter Video URL:"

    edit_line do |e|
      @url = e.text
    end

    button "Save to..." do
      destination = ask_open_folder
    end
  end

  button "Download!" do
    begin
      ViddlRb.get_urls(@url)
    rescue ViddlRb::DownloadError => e
      alert "Could not get download url: #{e.message}"
    rescue ViddlRb::PluginError => e
      alert "Plugin blew up! #{e.message}\n" + "Backtrace:\n#{e.backtrace.join("\n")}"
    end
  end

end

You MIGHT run into problems trying to download this yourself.
Some providers expect certain User-Agents when downloading (and youtube needs a few retries sometimes).

You can certainly give the shoes downloader a try, but depending on what you try to do in the end, shelling out to viddl-rb might not be the worst thing.
The "library"-functionality of viddl-rb is more of an afterthought :)

This is mainly a personal "hobby" program to make it easy to get YouTube videos as audio files.

Is it possible to call the CLI for viddl-rb from ruby?

sure, tons: http://tech.natemurray.com/2007/03/ruby-shell-commands.html