Originally Posted by itsabouttime
It must use the Caller-ID function, if you notice during the keynote demo, one of the messages on the screen was by "Unknown", meaning the caller was not in Steve's address book.
I noticed that as well. I hope wjp09 is correct and the iPhone will show the phone number when the caller is not in your contacts. Then the only time you would see "Unknown" is when caller id information is not available.