Bill Thompson has written a great whitepaper on considerations for Exchange 2007 servers running both the CAS and Hub roles.

This white paper provides information about the scalability of computers running Microsoft Exchange Server 2007 with the Client Access server role and the Hub Transport server role installed on the same server. It also provides planning guidance to help you select the appropriate hardware platform(s) for an Exchange 2007 deployment where you are deploying the Client Access and Hub Transport server roles on the same computer.

http://technet.microsoft.com/en-us/library/dd901772.aspx