The Density granular synthesizer will be available soon for the iPad under the name iDensity. In granular synthesis, the synthesizer plays sounds of small pieces of audio, or grains. You can record the audio by using the build in microphone of the iPad. The interface looks good:
Density is also available as a standalone version for Windows and Mac and a Max4Live version for use in Ableton Live. More information is available on the Density Pulsaret website.