I use an app called Auria to demo ideas...works very well. I also downloaded an app called audiobus that allows you to record other ios instrument apps to Auria.
I use Neo-soul keys (suitcase rhodes), a DM-1 drum machine app, and Animoog for bass. Great little demi studio.
Again, they connect internally on the ipad via audiobus.
I also bought an Iconnectivity midi interface that connects ipad to DAW and allows the ipad to communicate MIDI and audio data (yes, audio data as well) back and forth from your DAW.
The iconnectivity allows you to use your ipad apps to track to Reaper. Some are very good. Do all your MIDI in Reaper, it's sent to your ipad and the audio routes back thru the same cable to an audio track.
Audiobus also allows you to connect processor apps into the "signal chain"
Pretty powerful stuff.
Here's some links for ya.
http://www.wavemachinelabs.com/Products/auria
http://audiob.us/
http://neosoulkeys.com/
http://www.fingerlab.net/website/Fingerlab/DM1.html
http://www.moogmusic.com/products/apps/animoog-0