FB has the most complete contact list for a lot of people. It would be pretty cool if my phonebook ran off this and auto updates itself when people change their numbers.
To borrow a term I heard from Mark Suster though, it's a FNAC (feature, not a company).
Think about the supply chain, inventory management, retail distribution, OS development and so on that would need to go into this. I simply don't believe the business case for an FB phone exists. What's more, I don't believe FB believes it is either.
Some of these might be implementation issues, but I still don't think that even working perfectly it would be a killer feature for a phone.