There is some confusion on the licenses required when implementing Database Availability Groups (DAGs). Most of the times the confusion is caused by mixing up the OS license with the Exchange license. The answer is really quite simple.
DAGs use Windows Fail-Over Clustering. Fail-Over Clustering is only available in Windows Server 2008 Enterprise Edition (R2 included). So, each Mailbox Server that is DAG member should be W2k8(R2) Enterprise Edition. Stand-alone Mailbox Servers (i.e. non-DAG) can run on W2k8(R2) Standard Edition. Regarding the Exchange license, you can opt for Exchange 2010 Standard or Enterprise Edition. The difference is the number of databases supported running on one Mailbox Server; Standard Edition can host 5 databases, Enterprise Edition 100.
- DAG functionality?
- Yes: Windows Server 2008 (R2) Enterprise Edition;
- No: Windows Server 2008 (R2) Standard Edition.
- 5 databases or less?
- Yes: Exchange Server 2010 Standard Edition;
- No: Exchange Server 2010 Enterprise Edition.
Note that you can upgrade your Exchange 2010 Standard to Enterprise Edition when the situation requires. All that is required is the product key; re-installation is not required (see this TechNet article).