The next version of AppBuilder will be released very soon, most probably by the end of this month. In it you will find an experimental support for Cordova 3.7, that allows adding application resources for the new Apple devices. This is also implemented in the AppBuilder UI. However, iOS application will still be built against the iOS 7 SDK, using XCode 5. In other words, you will be able to produce applications for iPhone 6/6+, but you won't be able to utilize the new features from the iOS 8 SDK. The complete support for iOS 8 is planned a bit further in time, somewhere in January 2015.
I hope this helps.