What steps will reproduce the problem?
1. run ruby mget.rb 'http://youtube.com/watch?v=NrfUckNsGHM'

What is the expected output? What do you see instead?
[mulander@inferno mget]$ ruby 
mget.rb 'http://youtube.com/watch?v=NrfUckNsGHM'
[?] If you want to convert movies, you must install ffmpeg 
(http://sourceforge.net/projects/ffmpeg/)
[?] If you want to download mms:// streams you must install mplayer 
(http://mplayerhq.hu/)
/usr/lib/ruby/1.8/open-uri.rb:29:in `open': bad URI(is not URI?): 
http://www.youtube.com/v/NrfUckNsGHM' + query +' (URI::InvalidURIError)
        from ./lib/mget/youtube.rb:50:in `get'
        from /usr/lib/ruby/1.8/delegate.rb:296:in `each_line'
        from /usr/lib/ruby/1.8/delegate.rb:296:in `__send__'
        from /usr/lib/ruby/1.8/delegate.rb:296:in `each_line'
        from ./lib/mget/youtube.rb:48:in `get'
        from /usr/lib/ruby/1.8/open-uri.rb:135:in `open_uri'
        from /usr/lib/ruby/1.8/open-uri.rb:528:in `open'
        from /usr/lib/ruby/1.8/open-uri.rb:30:in `open'
        from ./lib/mget/youtube.rb:39:in `get'
        from ./lib/mget/mget.rb:259:in `getMovie'
        from ./lib/mget/mget.rb:138:in `run'
        from mget.rb:87
Tested under:
 Kubuntu (7.04) Linux
 ruby 1.8.5 (2006-08-25) [i486-linux]