The HTML5 video player gives you 1/2X, 1X, and 2X options as I recall. (the flash player doesn't though)Curious...
I saw a youtube video maybe 4 months back with an extra user control (next to the normal Change Quality button) allowing watchers to alter speed they view the video in.
At the time I figured it was an option to include for some video posters. Maybe it still is... never posted a video yet...?