Here's a VMware blog, one of a series, which provides a reference if you're looking to run Exchange 2010 Server on VMware http://blogs.vmware.com/performance/2010/05/exchange-2010-disk-io-on-vsphere.html
While the article make a valid point about increased RAM and improved response times (Exchange 2010 can use RAM as a cache for disk I/O), which seems to be the point of the article, I struggle to see how this is valid from a Disk I/O viewpoint. Testing has not been completed to validate this using a typical storage design for an Enterprise. A heavily revised storage architecture in Exchange 2010 gives a huge reduction in IOPS http://blogs.technet.com/ucedsg/archive/2009/05/11/what-is-new-with-exchange-2010-storage.aspx
To genuinely validate running Exchange 2010 on vSphere for an Enterprise, further testing would be required. Here's why;
a) FC disk is very expensive compared to DAS/JBOD/SATA which is supported by Exchange 2010 now. Unless you have a large fibre-channel storage available, with lots of disk available, then there are arguably more appropriate storage solutions available for Exchange 2010 now.
b) Repeat the testing using 2GB and 5GB mailboxes profile on SATA/SAS storage. (To be honest, 5GB isn't that capacious these days, 25GB isn't unusual now).
Why a user profile of just 100MB mailboxes in this test scenario? Exchange supports mailboxes of many GB now, mostly due to Enterprise data/retention demands.
I agree with the conclusion stated in the article
Assigning more RAM to an Exchange 2010 Mailbox Server VM can result in decreased IOPS and faster response time. This makes disk requirements for Exchange something that can be changed based on each environment. The strategy could be to support more users with the same number of disks or use fewer disks to support the same number of users.
I'm just not sure it helps you with disk I/O.