We recently deployed a new vSphere 5.0 U1 HA-enabled cluster. I noticed that a couple of times on an otherwise healthy cluster, after taking a host out of maintenance mode, the last HA event for the host says "The vSphere HA availability of this host has changed to Unreachable" as opposed to it becoming a slave. The fix has been to disable and re-enable HA on the cluster. Trying to figure out why this occasionally happens. Known issue?
As a side note - Since this does not produce an alert and can easily get missed, I wanted to setup a custom alarm for it. Setup the custom alarm using "The vSphere HA host availability state changed", but not sure how exactly to filter to only see the "unreachable" events.