Obviously the watch itself could not be a monitor as it lacks the necessary sensors.
Could it interface with existing HRM sensors? Not yet. From what we've been told, although the chip inside the Pebble is BT4.0 compatable, the Bluetooth stack being deployed on release does not enable that functionality.
When the stack is updated, and the API extended to allow it, then it should be possible to pair Pebble with a BT4.0LE HRM.
In the meantime, it may be possible to pair a HRM with your phone (as usually happens), and then hopefully when Runkeeper integration comes they will push heart rate data to the pebble (Runkeeper supports heart rate moniitors).