With Geco, an app for the Leap Motion Controller, synthesizers can be controlled with gestures, like shown in the video below. This works with MIDI. Hopefully the latency will be low enough.
The Leap Motion Controller is expected mid-May for PC and Mac. This controller is similar to the Kinect for Xbox. Apps for Leap Motion will be found in the Airspace app store, which will be launched in July. The price of Geco is $ 10.