I'm thinking about saving youtube flv videos with the name of unique id. If user would input the link, which's unique id matches to the file in server, the file would be converted and given for the user with the song name from youtube.
For example, user inputs http://www.youtube.com/watch?v=222kQyGGoWg
It takes the id 222kQyGGoWg and checks if there's a 222kQyGGoWg.flv file in the directory /videos/ ,
if there isn't, it downloads the file named 222kQyGGoWg.flv, then converts it to /mp3/ with named song_name.mp3 ,
if there is, it converts it to /mp3/ with named song_name.mp3
the minuses are that you would need to keep the flv files in your server, but you would save a lot of bandwidth because the service would need only to convert the existing video and to give users the mp3 file which size is small