downside of just an NFC chip is that it transmits one message at a time. so something in your arm without any 'processing' ability will end up just transmitting a private key. allowing any receiver to be the signer.
a better concept is either wait for microchips to have the power to see an unspent, sign a tx and send out only the signed tx, to be implanted. or make a device that can do it .. like a wristband.. that doesnt need to be USB cabled to a PC and doesnt need browser plug-ins/desktop clients to manage the receiving/sending.
that way the private key is always protected and also you dont need a pc for your own use to send a tx.
also you dont need something implanted by your doctor.
however.
i dont see an implant right now being a payment mechanism itself.. i do see an implant in the wrist can be locked to a password so that your wristband only functions when you are the one wearing it.