Alex Fontana over on the VMware blogs posted a great article on choosing between virtual disks (VMDK files) and Raw-device Mappings (RDM) when virtualizing an Exchange server. Check it out here: http://blogs.vmware.com/apps/2011/11/virtualized-exchange-storage-vmdk-or-rdm-or.html
I have deployed virtualized Exchange servers on VMDK files in ESXi 4.1 environments without any issues. I've also used the "In-guest iSCSI" method he talks about in the article, which is one of my personal favorites. The In-guest iSCSI method is what I use for Exchange servers that reside in my budget lab along with the FreeNAS based Virtual SAN I demonstrated in that series.
For production environments, I've stuck with either RDM, or the In-guest ISCSI, depending on the SAN architecture. I'll definitely consider VMDK from here on out.Exchange 2010, VMWare