For details about the new feature take a look at Frank Denneman's blog post vSphere 5.1 vMotion deepdive
You cannot vMotion (life migrate) VM's across Datacenters, so this will not work for migrating VM's between different vCenter instances.
In case you need to move a VM to another vCenter environment with the least downtime, you may take a look at vSphere Replication which is also included in most editions of vSphere 5.1.
André