I figure it is safe. But I also know there are enough bright people here to think of any loopholes.

The only connection from the modem to the outside world is via ethernet packets being transported over ATM. Ethernet packets are strictly routed point-to-point by MAC address. It is not possible for an ethernet packet to be sent over the phone line (ATM) unless the source knows the MAC address of something on the other side.

In the modem, the only way this happens is within the envelope of the PPPoE session. Packets arriving at the modem without a PPPoE wrapper simply cannot be routed.

That's my understanding.

Cheers