Already released but still worth mentioning is Akai's Synthstation 49, which is a synth controller especially designed for the iPad. Featuring a four octave keyset, nine mpc style drum pads, transport controls and support for CoreMIDI apps.
The Synthstation has a USB for receiving power and controlling MIDI software on your Mac or PC. You also get two professional 1/4" stereo outputs with volume control to connect mixers, speakers and recording equipment. Street price is 199,- $.