Hi Alex,
You should always create LUNs using SnapDrive - unless there are no plans to install it, but this would exclude SME, too.
You can't use VMDKs for Exchange, but you still have two choices:
1) RDMs - it used to be the case that SnapDrive only supported physical mode, not sure if that's still the case, but definitely physical is a safe bet
2) in-guest iSCSI initiator - could be useful if there are plenty of LUNs on the same ESX cluster, this way you avoid hitting the per cluster LUN count limit, as all traffic bypasses ESX stack.
Regards,
Radek