I currently have an implementation running with NFS datastores on NetApp Filers.
So this leaves two choices for Exchange 2007 mailbox server with a 500GB database:
1) Just Vitualise to the NFS datastores
2) Create LUNS and map iSCSI from within the VM to the LUNs thus allowing the use of snapdrive and snapdrive for exchange. This would have to use the same vSwitch as the NFS vmkernal port do to number of NICs on the hosts and the NetAppfilers
I like using the iSCSI initiator inside the virtual machine for Exchange. Having the ability ot use the native SAN based software against the LUNs makes a lot of sense. This is even more important if you are planning on replicating Exchange data to another location. Most SANs also have software that can run eseutil against the snapped LUNs to ensure the Exchange database is functional.
Exchange 2007 is less dependant on disk performance than Exchange 2003 you have more options on where to locate the stoarge. My vote would be to use the iSCSI initiator. That also makes clustering your Exchange environment easier, though the VMware support policies on MSCS clusters leave much to be desired in any configuration.
Matt | http://www.thelowercasew.com | @mattliebowitz