One of the no-brainer feature requests for Siri since day one has been to be able to type to it. For the times that Siri does not understand what you were trying to say, having the ability to just type something for Siri to do would make things far easier. Plus, should you be in an environment where you cannot talk aloud, it would still be faster to type to Siri than to do the entire action by yourself.
Finally, in iOS 6, this appears to be a reality. After dictating something to Siri, just tap the speech bubble to bring up a keyboard to edit your text.
Now, you still have initiate by dictating something — even if it's just the word "Um". You can't just start up Siri and type. But even still, having the ability to edit Siri entries is still a great thing to have. You can fix names and places that Siri refuses to recognize, and that's a huge deal.
Plus, I would have to imagine that Apple is sending this corrected data back to their servers to learn from Siri's mistakes. This could make Siri incredibly more accurate by learning from user corrections. This all makes a tremendous amount of sense, and is quite useful.
As informed by quite a few people on Twitter, this functionality has apparently been present since the later versions of iOS 5. I suppose just not many people knew about it.