Yes DAG is supported in the Exchange 2010 Standard edition
This information has until today been under NDA. But it’s great finally being able to share this tidbit with you guys.
So DAG is supported in both the Exchange 2010 Enterprise as well as Standard edition. But since the Standard edition is limited to 5 databases (versus 100 databases in the Enterprise edition), this is really only interesting to the SMORGs (small and medium organizations) out there.
Also bear in mind that since DAG still is partly based on the Windows Failover Clustering (WFC) component included with Windows Server 2008 Enterprise edition, this Windows server version is required both when utilizing DAG via the Exchange 2010 Enterprise as well as Standard edition.
MCM: Exchange 2007 | MVP: Exchange Architecture