it is legal to make mp3s from songs that you purchased. that is, if you go and buy a CD, you can rip the songs to mp3 and put it on your ipod. it is illegal to download these songs off the internet and not pay for them.
now, the question is, is it illegal to download these songs off the internet if you bought the CD?
no- as long as you own the cd you own rights to have copies of the songs. the whole issue with p2p downloading is that the majority of the downloaders do not have the cd and the associated rights to backup and thus are committing copyright enfringement.