vpsphere 4.1 onwards actually vmware introduced ATS..that is when ever a file is changed in VMFS. it will lock the entire metadata of the VMFS..that is true upto vsphere4. With 4.1 now if the array is VAAI capable.. it will lock certain portions of the metadata corrsponding the file which is changed. but in 4.1 it is partial.. and not fully ATS.
but now in vsphere5 it is fully ATS if the array is VAAI capable.. and it the array is not VAAI then SCSI reservation will be used.
So the correct term to use is metadata locking.. not the filelevel locking...file level locking is already there... only one ESX can take the owneship of that VMDK... metadata locking and filelocking is different.
refer the below link for moreinfo