This is something that I stumbled upon last night and put in the iOS exporter owner's lounge - I've seen a couple of people asking how to allow iPod music to play while a game is running. It requires a small change to some of the exported code - you need to go into CSoundPlayer.m and change lines 47-48 from this:
AudioSessionSetProperty(kAudioSessionCategory_SoloAmbientSound, sizeof(sessionCategory), &sessionCategory);
This will let any running iPod music continue in the background, together with your game's sound.
AudioSessionSetProperty(kAudioSessionProperty_AudioCategory, sizeof(sessionCategory), &sessionCategory);