One of the new features in the upcoming iPhone 4s is the Siri "assistant."
I don't know if this will be the world-changing feature that Apple touts - Android has already had much of this functionality. That said, reports such as those at Ars Technica indicate that it will be at least as well integrated, if not better, while allowing noticeably more natural speech.
One complaint about speech-based control and dictation systems, even the excellent software by Nuance (Dragon Dictation, etc.), is this: how does a computer, often in an office environment, distinguish between you and the guy next to you when he says "delete files."?
Plus there's the whole "who wants to look like a dork talking to their computer" angle.
One huge advantage that speech-to-text and voice command have on phones, as they approach the sophistication of the Dragon line of software, is that these are devices we are used to talking to all the time anyway.
Just food for thought.
If the Siri feature - which isn't just voice recognition but is also a context-aware parsing and interactive answer system works anywhere near as well as the examples in this video, then I think we're looking at a game changer. Maybe not today, but like I said, we already have a device we talk to, that often we can't conveniently put our hands on it.