There’s a cool new feature added to vSphere 4.1 for HA. If an error occurs you can easily check what the issue is by going to your cluster and clicking the “Cluster Operational Issues” line on the Summary tab.

If there are no issues the screen will be completely gray. I forced an issue though so you can see what is shows. Note that is also shows the “Role” of the host and in this case it is a Secondary Node!