The button does what you would expect - reads your comment back to you. It also has turned out that the way Youtube requests and provides the audio data is very simple:
http://www.youtube.com/preview_comment?q=TEXT STRING HERE&a=&s=&l=
You get back an MP3 file - any media player should play it fine! Now we all can ask the question:
http://www.youtube.com/preview_comment?q=How%20is%20babby%20formed&a=&s=&l=
Edit 8/10: Here's some of the links from the rest of the Internets, starting to take notice.
Youtube Community Help Forums: http://help.youtube.com/group/youtube-issues/browse_thread/thread/153d560d121e0677
Yahoo Answers: http://answers.yahoo.com/question/index?qid=20081006215031AAjpUVD
Digg: http://digg.com/programming/YouTube_nods_to_XKCD_by_providing_Comment_Audio_Preview
sorethumb at Livejournal points out some interesting things in the JavaScript, like the fact that we're all basement-dwellers and that the sex/age of the preview voice apparently can be changed: http://sorethumb.livejournal.com/832663.html
I've seen no confirmation from Youtube/Google (yet) that they were inspired by xkcd, but would seem likely, wouldn't it? Also, it seems that the Audio Preview button now shows up for every comment!





