The TNR-i seems like the most obvious candidate here - a perfect match for iPad technology. In some ways more easy to use than the original Tenori-On which caused quite a stir when it was released (pre iPad). There's also some MIDI control apps which can pair up with Yamaha's new iPhone/iPad MIDI connection kit the i-MX1 (price TBC) or presumably Line6 MIDI Mobilizer or any other Core MIDI compatible device.
Apps are as follows:
TNR-i a virtual version of Yamaha's unusual Tenori-On matrix instrument - £11.99
Faders X&Y Pad - a programmable MIDI controller for use with external MIDI gear - £2.39
Keyboard Arp & Drum Pad - prgrammable virtual MIDI controller with arpeggiator, keyboard, drum pads and ribbon controller - 342 arp patterns - £2.39
Voice Editor Essential for use with the Motif XF, XS, MOX, S70XS, S90XS - £2.39