That presentation was recorded by Confreaks [1] who let you download the video (and audio!) at different resolutions.

The Ruby community is really well-served by Confreaks [2] - they record and provide videos from many different Ruby conferences and have (over the years) expanded their remit to include Javascript, Scala and more.

[1] http://confreaks.com/videos/662-rubyconf2011-algorithms-is-n...

[2] http://confreaks.com/events