vSphere 5 introduces the vStorage APIs VixDiskLib_PrepareForAccess() and VixDiskLib_EndAccess() to lock and unlock migration for individual virtual machines.
From http://kb.vmware.com/kb/2008957: "This issue occurs because the entries from VPX_DISABLED_METHODS are not removed after a virtual machine backup task completes. When a VM-level backup begins, the backup system informs vCenter to disable Storage vMotion for that VM to ensure that the backups can complete successfully. If no backups are active for that VM when this error occurs, Storage vMotion did not get re-enabled automatically."
