With vSphere 5, the VMware HA infrastructure has been completely changed.
In vSphere 5.0 the new HA agent is “FDM” (Fault Domain Manager) and replaces the old AAM agent (from EMC Legato).
Other notable changes:
The old Primary / Secondary node concept has been replaced by a new and more simple Master/Slave node concept
A new Datastore Heartbeat function as been added (HA Architecture Series – Datastore Heartbeating)
No dependency on DNS
See this slideshare for more information: