Probably Dragon has been the oldest among all softwares trying to convert speech to text and here we find that they have come with an application for Iphone which can type for you with accuracy as you dictate. However one drawback which I find is that one need to be deliberate while speaking and secondly it performs poorly for Asian accent. Check out the video below
However one may like to check another application for speech to text which we consider as best based on the feedback. Please do your deliberation before going for it or you can have both applications on the phone.