Check the cluster sensitivity- http://www.vmware.com/files/pdf/using-vmware-HA-DRS-and-vmotion-with-exchange-2010-dags.pdf
Relavent section is 5.2 I believe. This seemed to work for us- at least, the databases don't flop anymore. Exchange guy said his indexes failed though- but they came back about 30 secs later.
See what you think.