A source for 9 to 5 Mac said that the feature is still being developed and might not be completed in time for inclusion into the next-gen Apple iPhone. Based on some SDK info, it would appear that Assistant would work similar to the Siri technology that Apple recently acquired. Assistant would integrate with the OS on your iPhone to allow certain tasks to be completed by uttering a simple sentence. For example, you could say, "Setup movie with Robespierre" and the app would check your location, offer you tickets to a movie from the closest theater, and send good ol' Robespieere an email apprising him of the information.
The service would be crowd-sourced and Apple will give users the option of sending their data securely to Cupertino so that the manufacturer can improve the service. The Assistant feature is expected to "talk back" to the user. And even when your Apple iPhone is locked, an option will allow you quick access to the feature.
source: 9to5Mac via Gizmodo
A leaked screenshot (L) and some SDK info (R) point to a feature called "Assistant" for the Apple iPhone 5