Application Ready Guide for Exchange 2010 from F5
With Exchange Server 2010 it’s finally possible to have a fully redundant solution with just 2 servers (as opposed to 4 servers with previous version). Nevertheless, it’s still not possible to use NLB (for the CAS servers, assuming collocation) together with DAG.
Although there are some more or less creative solutions, such as using virtual machines, the recommended configuration is to use external hardware load balancers.
F5 has recently released some Application Ready Solutions Guides for Exchange Server 2010: