This is simple and most of you might already know it..but i thought i'd post it anyway.
I tought of this my self while i was converting 3gp (mobile phone) videos into .avi
First what you have to do is download a youtube video using a grabber or a grabing site (like tubeg
(save it as .flv (you all know that:P ) )
Then you can convert it to .avi or mpg or w/e you want simply by using this site
(thedoors gave me this long time ago 
NOw when you browse and put the file it will give you a list of extention that you can cnvert it too..no mp3 is in there..unfotrunately you can only convert it to another video form....so you should voncert it to 3gp
Then you will browse the 3gp file again and you will see that there are 2 3gp types..one is for video and one is for music..choose the sound type (this way your file will be recongized are pure sound and no video in it) then you can choose convert to :mp3..and all done
that's it..pretty easy and pretty usefull since here are so many cool live conserts and music videos that i can't find on mp3
