In order to reduce costs, organizations like to co-locate server roles. From an Exchange Server 2007 perspective, probably the most common configuration is the combination of the CAS and Hub server roles. Although this configuration is fully supported, there are some doubts regarding the use of Windows Network Load Balancing (NLB) in a high-availability scenario.
NLB for CAS is supported. NLB for Hub Transport is not supported (with SP1 there are some scenarios supported, for further information please read http://technet.microsoft.com/en-us/library/bb124398.aspx). So what do we do?
This is really easy, all you have to do is define port rules so that only ports 80 and 443 (the ones used by CAS) are load balanced.
There is a nice article here at MSExchange.org with the details ofload balancing CAS servers: Load Balancing Exchange 2007 Client Access Servers using Windows Network Load-Balancing Technology.