You got it. Every BlackBerry has a specific PIN associated with it. BBs beginning with "2" are GSM/GPRS. 3 are CDMA. 4 are iDen. When the BB "registers" (via the radio), RIM knows what network it is on. So when traffic comes in for a specific BB, RIM handles the forwarding. So if I am in Kuala Lumpur, and my BB registers on Maxis Mobile, RIM passes the traffic to their network.

Obviously the international scenario doesn't work the same for CDMA devices.