Though it's still too early to gauge Siri's full impact, the short answer is that it certainly will impact SEO. A quick glance at the hipsters and their new iPhones will demonstrate to anyone with eyes that Siri is at worst a widely used novelty and at best the new way for finding things.
What kind of things? Well, as pertains to small business, people are finding local businesses with Siri. What's closest? What's best?
According to a recent Inc. article, small businesses would be wise to optimize "listings on Google Places, Yelp, Foursquare, Epinions... because more and more, [their] customers won't be hanging out on search engines." And in the short term, it's worth noting that Siri relies heavily on Yelp, so for companies that haven´t checked up on their Yelp presence lately, now would be a smart time to do so.
Before anyone gets too carried away, however, it's important to realize that Siri is only controlling a small percentage of user behavior at present. There is no reason to believe that Siri or any other voice recognition app will supplant Google's search engine results in importance to local businesses, as Google will be leveraged similarly soon enough.
But while the Siri craze takes off, you may as well hoist the business sail and see if the sales will follow.