First you need to hack the Phonegap lib and prevent it from hiding the splash screen after the webview is loaded.

1. Open PhoneGapDelegate.m and go to line 413:

  // -------------------------------------------
  // comment out these lines:
  // imageView.hidden = YES;
  // [window bringSubviewToFront:viewController.view];

  // ..and add these:
  [window addSubview:viewController.view];
  [window bringSubviewToFront:imageView];
  // -------------------------------------------

2. After phonegap and your sencha touch application is loaded call the helper plugin:

  // -------------------------------------------
  PhoneGap.exec("SenchaHelperPlug.removeSplashScreen");
  // -------------------------------------------