When you're talking about Siri; you need to talk about the virtual assistant piece separate from the speech recognition piece.

When Siri's ancestor was being worked on at SRI, it used a cousin of Nuance for the speech part. The PI of the virtual assistant piece of that program spun it out as a separate company, and was immediately snarfed up by Apple.

At that time, they switched speech recognition providers; but I don't know if Apple picked Nuance specifically.