It looks like Apple is working on several voice integration options for the iPhone. The latest evidence of this is a patent for text-to-speech and speech-to-text converter.
Here is more info from Patently Apple:
According to a new Apple patent application that was published this morning by the US Patent and Trademark Office, future iPhone’s are likely to provide end users with effective new ways of communicating in both noisy environments like a restaurant or even during a quiet office meeting without stirring a mouse. The system involves using new text-to-speech and speech-to-text converters as well as providing a means of sending prerecorded notifications to the caller if you’re unable to speak when answering your phone. I think that many will appreciate these new features and only hope that Apple could get these to market in good time.
Read the full report from Patently Apple.