it looks like its going to be a hardware feature. if the main CPU is off, it implies the radio circuitry and its CPU (the BBM) are still powered. give google this at least, the special new Bluetooth API will be accessible to whatever OS is alive and awake to send commands (even if I don't trust that "off" means "off"). the fact that its using encryption (that's too complicated to be made out of Integrated Circut logic) means its likely another software feature added to the BBM co-processor (it handles all radio tasks on the phone). this all but confirms the BBM (at least going forward) will still get power, be awake and have access to the (transmit (TX) and reseave (RX) functions of the) radios even when everything else is properly off.

EDIT: or it could be an abuse of a generic BLE beacon mechanism that's "just there for whatever the consumer would need it for". but if they are doing proprietary encryption like they claim, that's not really possible without updating the BBM's software to add another feature.