Saw some twitter activity on this tonight and I knowing we work with many NetApp customers on VMware vSphere I figure this has probably already come up or will soon.
First, what is Change Block Tracking? http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1020128
In a nutshell CBT is vSphere’s ability to track at a block level changes to virtual machines virtual hard disks. Applications that use the vStorage API can then access this information to do a backup of the changed blocks on a virtual machine.
So, if you’re using NFS storage, such as NFS from a NetApp storage array you will want to turn off this feature if you are experiencing this issue documented here. http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1031106