I was an early upgrader to iOS 6 and one of the best features is the update to Garageband for the iPad that lets it run in the background. I didn't realize quite how amazingly useful this feature was to me until making my last piece.
I have tons of synths on my iPad that I always underutilize because it is too difficult to play them in isolation, copy the audio and then reassemble it with other tracks. There are a few other apps that allow background playing such as BM2, but I don't have that one yet.
So, with Garageband in the background acting as an "aggregator", I can slowly assemble my piece track by track and have it playing in the background while recording overlay tracks in Animoog or Sunrizer.
Here is the piece I put together with Garageband, Figure, Animoog, Sunrizer and iVoxel:
It has a middle-eastern vibe to it - the warbling vocals are actually my voice pitched up several octaves with an LFO vibrato added to it.
The main drawback to Garageband as a collecting DAW is that you only get 8 tracks to work with. This piece takes exactly 8. If you need to go beyond that, Garageband lets you bounce together tracks and combine them - so it is a pretty nice workaround if you need it.
I will probably make use of this quite a bit until AudioBus finally hits!