Erik over at Humble Tune has updated his iPhone apps, Shapesynth and Sinusoid, with Virtual MIDI and background audio support. All in an effort to ready them for playing with his latest app, Brother. Which he introduces as, "a polyphonic, cleaner, brother of shapesynth."
Then after all of that hard work he made everything free!
Brother sounds pretty interesting. There is a mode that can read incoming tweets and then try to drive a sequencer based on them. It is feature-rich for such a simple looking app, with AudioCopy, Virtual MIDI, and SoundCloud uploads. Perhaps most impressive of all is the "slave" mode for Shapesynth to work with Brother. This is an Audiobus-Lite-like implementation that lets Brother record into Shapesynth's Shape Tape!