Boneyard,
In theory you could monitor access points via ping, but that goes back to days when access points were expected to have static ip addresses. These days, access points can boot with any ip address from any location and still come up when the network is dynamically changed around them. The controller monitors which ip address an access point has an maintains a heartbeat to determine who is up and who is down. That eliminates the management overhead to maintain a static ip address table offline. Changing the network that an access point is on is also simplified. Since the controller maintains the ap name to dynamic ip address mapping, as well as provide feedback as to if the access point is communicating with it, polling the controller via snmp or receiving access point up/down traps is the best way to monitor access points.