By now just about everyone knows that you can use Siri to take dictation on an iPhone 4S, but what you may not know is that you can also use it on a Mac. If you have an app on your iPhone that allows you to access your Mac's keyboard functions remotely, you can use Siri's dictation feature on the iPhone 4S to dictate text to your Mac.
One example of an iPhone app that you can use to dictate to your Mac is Mobile Mouse. The app already allows you to control your keyboard and mouse from your iPhone, but using the new dictation button on the keyboard, you can also dictate text to your Mac. (Reader Amalesh Panse pointed out, via Twitter, that Magic Mouse works fine with Windows too.) So long as you have a cursor inserted into a text field, you can use the dictation button on your iPhone's keyboard to use Siri to dictate texts directly to your Mac using Mobile Mouse or a similar app. Conversion into text happens rather quickly, almost as quickly as it does on the iPhone's native interface.
In practice the dictation is actually quite accurate; it does make mistakes, but I managed to dictate almost all of this post using Siri via Mobile Mouse with only a few adjustments. Apart from being an extremely cool trick, this feature could also allow you to bypass paying upwards of $50 for a product like Dragon Express, which does essentially the same thing (perhaps better).
The best part is, there are no settings that you need to tweak in order to get this to work. If you already have Mobile Mouse installed on your iPhone (and Mobile Mouse Server on your Mac), you're already able to use Siri to dictate text to your computer. Seeing words I've spoken into my phone appear on my Mac's screen as if by magic is one of those whiz-bang things that totally reminds me we're living in the future.
Thanks for the tip Rohan!