Click on the video to go to YouTube itself. Then go over to the box to the right of the video and click "More Info" next to the date added. Follow the link to the actual MP3, then download it to your computer and transfer it to your phone.
For those who don't know how to make MP3 ringtones, I use Mobile17.com. It's free and relatively easy to understand.
I just made the Murdertone my new ringtone on my iPhone. It's brutal.