Hi PaulS,
During use (not uploading a sketch) the microcontroller's listening to the brain thing, but not talking to it. It can therefore talk to the PC, but not listen.
From the brain library readme:
Note that the connection to the Neurosky headset is half-duplex — it will use up the RX pin on your Arduino, but you will still be able to send data back to a PC via USB. (Although in this case you won't be able to send data to the Arduino from the PC.)
Hope that clarifies.