Jon Briggs only found out that he was the voice behind Siri in the UK for Apple's new Iphone 4S when he saw it on TV.
Siri takes in questions, commands, requests, and meeting changes to mention a few items and then sometimes quite funny & cleverly replies with the most ideal reply.
For example "When is my next meeting?" Siri will then reply telling you "Your next meeting is at 10am next Tuesday".
Jon Briggs is the voice behind the assistant Siri that in fact was recorded over 6 years ago!