Recently I started to show a friend how to convert YouTube videos into mp3, and her first reaction she's get in trouble for pirating music.
Which got me thinking. I'm no attorney, and I'm sure the law here is arcane and probably muddy enough to even cause head scratching and debate among lawyers practicing more everyday law.
My take, it's okay to copy a song off of YouTube, as long as it’s only for personal use. The "fair use" allows one copy for personal use, just like when you record a TV show with your DVR, you are then creating one copy for your use. It does not become illegal until you start distributing that show, or music for profit, or passing on to friends for non profit use. Just the act of distribution is where it officially becomes illegal.
Where I think mp3 ripping crosses the line, or at least is legally challengable, is conversion services that also archive MP3s and store them on its servers for future downloads. So if a user visits the site and enters a URL that someone previously had, then instead of re-ripping the song, they could just go to its servers and give the user a copy of the MP3 that was already stored there. Which … is essentially a lot like illegally downloading music.
Now, whether mp3 ripping in any form is moral, is another question. The quality isn't that hot, I think most people are like me and do it for casual use and sampling, and that it leads to more licensed purchases, rather than to build a serious library.