Boy Genius and 9to5Mac have been digging deep into how FaceTime might work on iOS devices that don't have phone numbers attached to them -- namely the next generation iPod touch G4 and iPad G2. Since iPhone 4 uses phone numbers as "accounts" to route FaceTime connections, the alternative for iPod touch and iPad seems to be Apple ID and the associated email address.
You enter the email address, hit verify, check your email, and if it works -- boom -- you're good to go. Then something like Push Notification could be used to request a FaceTime session.
If iPod touch G4 and iPad G2 indeed get their front-facing cameras and FaceTime on, and if Android, Palm and others choose to implement the open standard Apple released, next year could be a very interesting time for smartphone users. Hundreds of millions of smartphone users.
My only question is why didn't Apple just use ID/email to start with, even on the iPhone, to keep everything standardized across devices?