If you drag your new video file into the iTunes window, iTunes will copy the file into its library and play the audio track. You can then discard the video of this file by selecting Convert Selection to... from the Advanced menu. People with golden ears will decry the quality, rightfully pointing out that you're compressing already-compressed audio, but it's a way to get free music from iTMS.
[robg adds: If you have QT Pro, you can use the QuickTime Player to extract the audio -- select File -> Export, and then in the Export pop-up, select Sound to AIFF. You'll be left with an AIFF file at the original quality level, which can then be converted in iTunes. You could also use a tool like Audio Hijack to capture the audio. These techniques are not unique to iTMS videos; you can extract the audio from any QT video clip this way. Note, however, that the quality of the audio in most downloadable videos is quite low; I did some head-to-head comparisons with songs that I own and have ripped, and it's not even close. The video-extracted versions sound like AM radio in comparison ... which is probably why nobody cares if you extract the audio from the freebies.]

