Just found this. Not sure if it runs on Windows Phone 7 too, but works like a charm on my Lumia 920. It's quite expensive, but after looking at the screenshots I decided to give it a shot.
One thing that bothers me though is that only 2 Siri-like commands are available at the moment. The meeting one, and the alarm one (which are by the way also accessible from outside the app), which renders it useless if you want to use it for tasks on your phone. That said, the general knowledge engine in it seems to work just great, and is worth the whole thing alone. I tried firing all kinds of insane scientific questions at it within the field of chemistry, physics, unit conversion and biology for school related work. This makes me suspect that it uses the same knowledge engine (Wolfram Alpha) as Siri does. You can talk to it apparently as well, as it responded to some of my general sentences or comments.
If you want to, I can add some screenshots of my queries so far.
As for speech recognition tech, I am not sure what technology is used. Might be the inbuilt Windows Phone 8 speech, or Google's speech API.
I posted this on WPCentral too, and they asked me to show it off on YouTube. So I made a short video review of it. The video is still processing though, so it might take a few hours for it to be up.