If you've never used Siri before, I can tell you that Siri was an "it". It was just an app. However, during the iPhone 4S demo with the enhanced Siri, the voice was definitely a "she", albeit a robotic female voice. But is Siri really a she?
See, I think we need to hold our judgment on that for a while. Siri started off as an app. A bunch of codes. Nothing masculine or feminine about that. And it was likely a bunch of dudes who programmed and created Siri. Apple decided to give Siri a female sounding voice in iPhone 4S but Apple could have just as easily went the other way.
In Star Trek: The Next Generation, Command Data, the Android Star Fleet officer serving on board the USS Enterprise D, who longed to be human decide to follow a trait that most humans have: to procreate. So in "The Offspring", he created another android in his own image, his child.
But he gave that android a name but allowed it to chose its own sexuality.
Siri will grow with added features. Its AI programming, I don't know if I am writing to call it that, will definitely become more sophisticated as years go by. I'm not saying that Siri will become sentient like the androids, robots, or programs popularized in science fiction, but it could gain a distinct personality that could cater to the needs of the users.
So, perhaps, it's too soon to call Siri a her just because of the female voice the iPhone team gave it. Maybe we should be allowed to individually decide if Siri is a he or she. Who knows, Apple could one day give users the option to personalize the voice as we can do now on Mac OS X.