Hi Simon,
Theres no app that I know of that can do that, but do you need to use iTunes to do it?
I use a small bit of software called ASD - Amazing Slow Downer. Its a fantastic little tool that will change the pitch and tempo of a tune. It also allows you to loop sections of an MP3 and save the altered looped sections to mp3. AND it has a karaoke function which attempts to cut out certain instruments - which takes a little tweaking but it is very useful.
Its available here:
www.ronimusic.com
I mainly use it for transcribing tunes, and where a solo is too quick, or out of tune, I can quickly manipulate it.
Oh, and also supports playlists.
I cannot recommend this little bit of software enough, and no, Im not on comission