I know that Microsoft now allow Exchange 2010 mailbox role to be clustered, therefore I just wanted to confirm that in the scenario of a 2 host hyper v cluster, you could have a single vm with the MBX, CAS and HUB roles installed and take advantage of the hyper v HA cluster service, negating the need for the DAG cluster. The benefit would be only a single exchange license is required.
Yes, Microsoft now supports hypervisor-based high availability mechanisms. They added this support in the middle of of last year.
That said, bear in mind that a DAG can be useful for far more than just local data center availability. With a DAG, you can better protect against database corruption and can extend availability to a DR site.
So, you might save the Exchange license, but you might lose some other opportunities.