Siri is crazy. Being able to talk to your phone like a real person is the stuff of Sci-Fi. Here's the catch: You won't be able to use it unless you've got an iPhone 4S.
Siri's built into a bunch of apps already, and the software runs on both the 4S and in Apple's data centers. Because the process is so CPU-intensive, it'll only run on the iPhone 4S with its newer A5 chip. That makes sense with how many features are packed into it, like learning your voice's natural cadence and understanding English, French, and German, but it's gotta be a little disappointing for iPhone users who aren't up for an upgrade just yet.