Or, you can use this freeware app...

http://www.macupdate.com/app/mac/26798/iextractmp3

Or: (I use Snow Leopard)

1) In Safari open the Activity window
2) Go to the Youtube site and start playing the video.
3) In the Activity window identify the largest file that is downloading from the Youtube site.
4) Hold the 'Alt' button on your keyboard and double click with the mouse on the line with the largest file in the Activity window.
5) A file called "videoplayback.flv" should start with downloading.
6) Open the downloaded file in QT X (QT X should open *.flv files, otherwise use Perian)
7) Use QT X to export the file to an Apple format, preferably mov.
8) Fire up your Snow Leopard install DVD, and install the Quicktime 7 package. It can be installed and be used besides QT X.
9) Open the mov file in QT7, and use QT7 extended export options to export the movie file to a desired audio format.

All available and 'free' software for any mac user, it is little elaborate, but gets the job done for free with standard software.
Maybe this could be automated, I am not a script wizard.

2cts

I second the motion of using iExtractMP3.
I use nearly all of the other tools mentioned in this thread. They are all great, but none of the methods or solutions mentioned are as easy as iExtractMP3.