Found some very helpful information over on Duncan’s website Yellow Bricks.com regarding the management network and jumbo frames, on vSphere 5.

http://www.yellow-bricks.com/2012/01/20/no-jumbo-frames-on-your-management-network/

For our reference, I’ve copied the content below:
“Another thing to check if you experience this error is to see if you have jumbo frames enabled on the management network, since this interferes with HA communication.”

This is document here in a note: http://kb.vmware.com/kb/2006729

To make it crystal clear: disable jumbo frames on your management network with vSphere 5.0 as there’s a problem with it! This problem is currently being investigated by the HA engineering team and will hopefully be resolved.