I’ve heard a couple of queries recently concerning Windows NLB and the CAS\HUB combined role. The point is that you should use NLB to load balance network traffic and you can therefore effectively de-couple HUB from CAS. For the Hub Transport server role (since Exchange 2007 SP1) the following is supported:
“Load balancing of inbound SMTP connections for POP and IMAP client connections to the default Receive connector named "Client <Server Name>" that is created only on Hub Transport servers” AND “Load balancing of inbound SMTP connections for applications that submit e-mail to the Exchange organization.”
But as a quick caveat to that you shouldn’t attempt to use NLB to distribute connections for internal routing between Hub Transport servers. There’d be no point in attempting this anyway of course since Exchange makes a pretty good job of it.
As far as the CAS role is concerned since RTM using NLB to load balance has been supported. I have generally seen NLB used to load balance internal access to the CAS ports with ISA Server used to load balance external connections.
And once more by the power of live search…
What Is Network Load Balancing?
How Network Load Balancing Technology Works
Network Load Balancing Clusters
Quick Tip: Configuring Network Load Balancing (NLB) on Windows 2008 for Exchange CAS Servers…
Network Load Balancing: Frequently Asked Questions for Windows 2000 and Windows Server 2003
..and to chuck something else into the mix I have seen NLB used to load-balance connections to a combined role server on Hyper-V. Will be a pretty good solution for smaller deployments I’d expect.