From the phone side it's intuitive enough: call the email address associated with an Apple ID and you get the computer, call the phone number and you get the iPhone. Conveniently, your computer will "ring" even if the app is closed, though you can turn this feature off in System Preferences if it unsettles you. Hopefully Apple takes this "beta" moniker seriously and has some extra features to add before it releases the final version of the app. We also wouldn't mind seeing this folded into iChat proper -- why have two first party video chat methods? Either way, computer-to-phone conversations are here to stay. The payoff? The psychedelic experience of viewing four Paul Millers simultaneously.
Update: Want a special little bonus? You can apparently video chat with iChat and FaceTime simultaneously, though we tried to add Skype as a third conspirator into the mix and failed (there's a pic of our experiment after the break). Oh, you should also know that there's apparently no way to shut off the video in FaceTime outside of quitting the app or physically covering up the camera. EXCLUSIVE: Is Steve Jobs Watching You Sleep? More at 11.
FaceTime for Mac hands-on