I don't want to get back into this because it's way too much to think about BUT
The iPhone is the account. And your "buddy list" is anyone in Apple's servers who has an iPhone 4.
As I already explained you, this only works HALFWAY.
The first half is FINE. You buy an iPhone and activate it, in Apple Servers your iPhone (its SERIAL NUMBER and your itunes account) is marked as FT-capable. Fine.
Apple Servers hold a list of those FT-capable devices. (=Serial Numbers and itunes accounts of the owners). FINE. That's the buddy list. An alphabet soup buddy list. Not people, but serial number of iDevices.
But THEN WHAT?
How do people call you? How do their iPhones know how to match "mloffa the nice dude" from Lisa's Address Book with "Device XXXXYYYYAAABB" in apple servers?? If she taps "mloffa the nice dude" in her iphone address book, what is her iPhone supposed to do? Pick a random FT-capable device from the Apple Servers? You should give her some kind of CODE or ACCOUNT or ALIAS in advance. So she register it under your contact.
That's the problem.