Did you know that you can record directly to your iPad using a USB audio interface or USB microphone?
By using the Apple USB Camera Adapter you can plug in any ‘class compliant’ USB audio device and record directly to your iPad using your favourite audio app. (It’s worth noting that you can get the Camera kit slightly cheaper at Amazon! )
Here is a short video that shows the camara adapter and a USB microphone in action.
The stipulations to getting this working are that the interface has to be class-compliant (will work without its own drivers under Mac OS X). It should also require less than 20ma of power, if the device draws more power than this you’ll need an external supply to provide power to the interface or microphone.
Here is an Ableton forum with a list of some class compliant devices that have been found to work.
I also found a page on the website for the MIDI Touch software showing some devices known to work with MIDI.