The database and the transaction logs need to be on separate LUNs to support VSS hardware provider snapshots that SME uses.
Exchange databases can share a transaction log LUN and SnapInfo directories can be located in the transaction log LUN.
Exchange 2010 DAGs provide Exchange high availability through having multiple copies of databases. So if you have a 2-node DAG, you will create a copy of each database on each node. If you lose a database, Exchange will automatically start using the surviving copy without disruption to users (if they are using Outlook in cached mode).
I don't know how big your databases are, but I suggest that you use a few large databases instead of a lot of smaller databases. With SME, you can leverage a large database design because the time to restore a 2TB database is basically the same amount of time to restore a 200GB database. Combining the benefits of Exchange HA (multiple database copies) with the rapid restore capabilities of SME/DataONTAP, you are reducing your risk.
In addition, a side benefit of using fewer databases is that your backups will go faster.
Thanks,
Mark