The vSphere 5 release brings some really cool new things into the mix. High Availability has been completely revamped and enhanced. Forget everything you remember about "aam" because it's being replaces with Fault Domain Manager.
VMware is taking a page out of it's own book by extending the VMware heartbeat signal from hosts pinging each other through VMkernel ports to now being able to keep in communication through datastores. If you're not familiar already, vCloud Director cells keep in contact with each other through datastores so it's another layer of resiliency.
During my testing of vSphere 5, I've found a cool little nugget called "all paths down" which assist hosts during failures. This feature will send a pause signal to a VM during a host failure and if the host resumes operations in a reasonable amount of time, the VM will resume operations. We're all used to losing storage or losing hosts and experiencing BSODs and resets, but this new feature allows a VM to pause its I/O and connections for a matter of time until the connection has reset. I've been able to use this feature in my own home lab (a few times) when my foot accidentally shut off my synology NAS. The NAS hosts all my VMs and was completely turned off. I immediately turned my NAS back on and once the NAS was back up and functional, my VMs never missed a step. vCenter, AD, etc were all sitting on the same screens as they were before the environment got shut down.
I wish I had more information explaining this technology, but I've seen it up close and personal, and it's pretty awesome.
Want more HA feature goodness. Check out vSphere 5.0 is released with new HA features from VMware Uptime.