I know I’ll get splattered with shaky eggs for even suggesting this, but what the hell!
ABRSM (the UK music examining body) has free software which you can download to shift speed to allow you to play along with stuff at a speed of your choice. It will also shift in semitone steps.
This is a PC prog rather than an ipod app but if it sounds useful it’s at http://www.abrsm.org/students/speedshifter
I’ve very occasionally found a use for this stuff - when learning something with complex harmonies (for example, Tarantella by Rudiger Opperman)