Alesis introduces the iO Mix. With this mixer and recorder for the iPad you can record four instruments or vocals simultaneously via the combinated XLR-1/4 inch inputs in apps that support CoreAudio, such as GarageBand. The iO Mix also has a video output for video playback during live performances. Unfortunately, the device has no MIDI in / out.
Are you looking for a more compact solution wíth MIDI in / out? Then take a look at the iO Dock. It has two combinated XLR-1/4 inch inputs for instruments and microphones.