According to Boy Genius, an upcoming iOS 4.1 beta code suggests "FaceTime" will become available for non-iPhone devices like iPod Touch and iPad. Here's how FaceTime will work on non-iPhone devices:
"Your Apple ID will be registered with your device/s, and that'll allow other Apple devices to start a FaceTime call using your email address. Apple will also reportedly make use of push notifications to deliver these incoming FaceTime connection requests as there's a push notification detector tied into FaceTime frameworks in new iPhone OS. We're not sure how Apple will handle an iPod touch trying to connect to a FaceTime session on an iPhone as it seems Apple would still require you to use iPhone's phone number, at least that's how it seems currently. In all reality though, we're pretty confident Apple will make a straight forward and unified way of talking to every eventual FaceTime-enabled Apple device or computer, and it might be an Apple ID, or possibly something else down the road."