If you're referring to a carrier's zero interest payments on the full price of the phone, that's just smart. No penalty for borrowing. If there was interest involved, it gets fuzzier.

I'll amend this by saying that there's also another huge requirement for it to make sense: The new iphone must be necessary for any of this deal to make sense. If it was impulse bought because of this financing offer, then the financing has cost you >$1k