Doepfer

actually it's already very possible with an iPhone (I do it 
frequently) and of course with an iPad you will have a bigger surface

_on the appstore find either "mrmr", "Fantastik" or "TouchOSC" (all3 
are have strong and weak points - on fantastick you can build your 
interface, whern on touch OSC you organise pre-made sets of 
objects........ etc)
_create a small network between you iPad/iPhone and your computer - a 
wireless one, really easy
_play your iPad/iPhone, and receive the data on your computer (most 
of them send it as OSC, other as just "data" on a network)
_I personnaly use maxmsp to receive the OSC, but there a many ways - 
including free apps.
_convert it to MIDI (for me , max again, but other solutions do 
exist, incuding free ones)
_send the MIDI out

really easy to set up, works great - i did use it many times live on 
stage, no problems, but a lot of fun

best

kasper
-- 
  Kasper T. Toeplitz
noise, composition, bass, computer
http://www.sleazeArt.com

http://www.myspace.com/sleazeart