Microsoft is shifting focus on Load Balancing solutions. During TechEd 2010 in Berlin it was announced that Windows Network Load Balancing (NLB) is no longer recommended (but still supported) and that the recommendation will be to use a hardware load balancer.
Another new recommendation is to use combined Exchange server with all three Server Roles, i.e. Hub Transport, Client Access and Mailbox Server Role when possible.
When using a Database Availability Group (DAG) you have to use a server outside the DAG as a File Share Witness (FSW). Normally an Exchange 2010 Hub Transport Server is recommended for FSW usage, but when using a two node DAG each with three Server roles an additional Hub Transport Server might not be available.