Apple picked up mobile assistant app Siri quite a while ago, but only at today's event did we finally see why. The company is relaunching the app as official, though it will keep the name Siri. To access it, all you'll have to do is hold down the home button, and then a little purple mic will pop up, listening in to whatever question you ask. From your question, Siri will then find any and all pertinent information, including data about the weather, stock and clock information, and even searches for local restaurants and businesses. You can ask for directions and get them right away, or even get messages read off to you via Bluetooth, and create calendar appointments and replies right inside the Siri system.
The whole thing is very impressive, and if it works as well as Siri did, Apple's phone has yet another killer feature on it. Unfortunately, Apple said that Siri is "the coolest feature of the new iPhone 4S," so presumably you'll need the new handset to make use of the feature. We'll double-check on that going forward, but all indications are that Siri will be 4S-only.