Here's another sign that Siri may be coming soon to the Mac: it already seems to know what Finder is.
In a blog post, Brian Roemmele writes about a curious Siri response he came upon. If you say, "Siri, open Settings in a new window," Siri will respond by saying, "It doesn't look like you have an app named Finder."
Siri seems to know how Finder is used
What's particularly surprising is that Siri references Finder unprompted. That suggests it's aware of Finder (OS X's file browser) and knows what commands require it. Apple has put info into Siri early in the past, so it's entirely possible that knowledge necessary for its launch on the Mac has gone live a little too soon. The Verge was able to replicate the response.
Siri's coming launch on OS X was first reported by 9to5Mac in February. Later, MacRumors published leaked images of Siri icons in the OS X menubar and dock. Taken together, it certainly seems like we're going to see Siri running on the Mac at Apple's developer conference next week.