Hey Luc,
Just wanted to thank you and the original poster for this thread.. I have a task to migrate some servers from one SAN to a new one and 2 of the VMs have 27 VMDKs which reside on over 10 datastores. I was going to migrate them by hand using the wizard, but when I saw this thread I realized I could script it AND could run it with the -runasync switch and do them all at once with the VM powered off. 
Thanks!