Delora Software's gbTouch--iPad control surface for GarageBand
"Delora Software's gbTouch is an iPad app that seamless integrates many pro-level control surface features into GarageBand (Mac OSX version). It offers full transport controls (including a "jog strip" for easy play position movement), track mute/solo/rec, track names, track volume faders, and color-coding to indicate whether the track is a "real instrument" track, or a "software instrument" track. It uses Wi-Fi to connect to the Mac, giving you the freedom to control GarageBand from anywhere in your studio.
gbTouch's multitouch UI automatically adjusts to your preferred orientation (landscape or portrait)."