About halfway into the video below, iPad as a controller with Omnisphere is shown in some depth. At around 16:30, he shows the Orb, which was added to the latest Omnisphere, controlled with an iPad. The Orb can be used with a mouse of course (how I use it now), but would arguably be more fun with an iPad and almost certainly easier to work with if used in live performances. They could also do some things with multi touch that wouldn't even be possible with the mouse, but they might be avoiding tablet-only features for a reason.
http://www.spectrasonics.net/video/i...ack.php?id=131