Occasionally it is necessary to remove a host from management and then re-add it back in.  This will result in the loss of any metadata associated with that particular host.  Such metadata includes the custom properties, descriptions, tags, owner, cost center, etc. If it is 1 or 2 VMs, its not a big deal to add them back, but when you are talking about a cluster with 200 VMs it is quite an effort. Michael Michael has posted a great blog that outlines exactly how to restore the metadata through the use of powershell scripts.

http://blogs.technet.com/b/m2/archive/2010/04/16/saving-and-re-applying-the-virtual-machine-metadata-in-vmm.aspx