It seems like Apple is building Facetime up to be a new "phone feature" like calling and text messaging, ie. a service that goes from phone number to phone number. Remember, Facetime is an open protocol that other phone manufacturers can use, so it won't just be "iDevice to iDevice" either. I think Apple expects other manufacturers to follow suit and implement it so that eventually it will just be the ubiquitous features that anyone with a cell phone has access to.
Unfortunately, that means we won't get it other internet devices such as iPod Touch, iPad (without 3G), or regular computers with iChat. There's no "phone number" for Facetime to connect to. We already have video chat solutions. Apple is trying to do something entirely different here.
HOWEVER, I do think that 3rd party video chat apps will show up for the iphone, and those will allow you to video chat with people on any platform just like you'd expect. It just won't be "Facetime" by name.