According to a report from 9to5Mac, Apple is testing voice dictation on future devices where the service would take place locally, as discovered within strings of code in iOS 7 beta by @hamzasood. Devices with this feature are apparently being tested internally at Apple:
Based on what Apple is testing, future versions of iOS could allow an iPhone, iPad, or iPod touch to process and convert speech from a user right on the device itself. This would speed up the process considerably, allowing emails, text messages, notes, and web searches take place via voice quicker than ever. The functionality is currently not active in public-facing builds, but we are told that devices internal to Apple have the function up and running.
By having dictation happen locally on devices themselves, it would not require an internet connection and also serve results much faster. Currently, Siri Dictation and search queries are still being criticized for being too slow, especially compared to Google Now.