A new app from developer Avatron has found a way to use a Siri-enabled iPhone to dictate to Macs on your network using your voice. For now it's limited to text input — there's no voice control of other applications just yet — but considering there's no public API for Siri, this is still impressive. The iPhone app requires you to install a software receiver onto the Macs you wish to use, which then broadcast their availability around your Wi-Fi network. Once you've selected your Mac and used it to open up an app like TextEdit or Microsoft Word, you can speak your magnum opus, it'll be run through Apple's servers, and then pasted into the waiting desktop application.
Although other apps like Nuance's Dragon Microphone have been able to do similar things before now, Siri has been recognized as being far more accurate and adaptable to different speech styles than other voice recognition engines. Air Dictate costs $0.99 and you can grab it from here.