Things have changed a little in VI3 but vmware have released a document on how to do it. It seems you have to zero out the disks before sharing them.

You must zero out the disks you use with a cluster in a box scenario. You have these

options:
  • To create and zero out in one sweep, type the following at the command line:

vmkfstools -c <size> -d eagerzeroedthick

/vmfs/volumes/<mydir>/<myDisk>.vmdk

  •  To zero out an existing disk, type the following at the command line:

vmkfstools [-w |--writezeroes] /vmfs/volumes/<mydir>/<myDisk>.vmdk
  •  If you wish to save time, and don't mind having stale data as it exists on the

physical medium in your virtual disk, type the following at the command line:

vmkfstools -c <size> -d thick /vmfs/volumes/<mydir>/<myDisk>.vmdk

http://www.vmware.com/pdf/vi3_vm_and_mscs.pdf