The patent, which was first spotted by Apple Insider, describes how a desktop version of Siri would work.
For instance, you could summon Siri by performing a specific gesture, which would involve holding a key, clicking on something within the OS X interface, or forming a pattern on the tocuchpad. (Mac users can already use the dictate feature in OS X 10.8 or later by tapping the function key "fn" twice, but you can also customize that particular shortcut.)
According to the patent, once you activate Siri, the virtual assistant would be smart enough to know whether you're asking a question or dictating speech to be translated to text form (Nuance's latest version of Dragon can do this). When you issue a command, Siri would be able to put your instructions in context based on what you're doing.
But perhaps the most interesting feature mentioned is the idea that Siri could act as a "third hand" for Mac users. In other words, she'll be able to help you complete a task when you ask for her help. From the patent's language, this sounds like you can assign Siri a task to work on in the background while you finish up your work.
Of course, this doesn't provide clear evidence that Siri will come to the Mac environment. Apple files new patent applications all the time - many of which never turn up in the company's products. Still, it's interesting to imagine how Siri would translate to the desktop if Apple ever chooses to do so.