This advanced HA option is my new best friend. I'm sure there are those of you out there preparing your hosts for the ESX to ESXi migration. Enhanced network compliance was introduced in vCenter 2.5 u2 to help ensure correct cluster heartbeat networks and "das.bypassNetCompatCheck" was introduced in U3 allowing you to bypass this network compliance. Brian Atkinson has a good blog on this as well.
Why am I bringing this topic to you? The advanced HA option is GOLD in a cluster migration from ESX to ESXi. On ESX, you have a service console for heartbeats and management, but in ESXi you use a vmkernel for management traffic now. If you mix ESXi host and ESX hosts in the same cluster, HA will error out because of the mismatched vswitches.
So before you automate the upgrade or manually install your cluster from ESX to ESXi, set "das.bypassNetCompatCheck = True", this will allow HA to function during a staggered upgrade path. When the upgrade is complete, remove this advanced HA option.