I was asked this by a school district in Minneapolis that was deploying Exchange 2010. I have seen this becoming a popular deployment option since CAS is proc heavy (3 proc cores for every 4 mailbox proc cores) and Hub is not as proc heavy (1 proc core for every 5 mailbox proc cores with Antivirus running). Why not combine the two roles to maximize your server hardware? The question becomes what are the sizing guidelines for this?
We just published a combined Hub/CAS hardware guideline:
Combined Hub/CAS core processor to Mailbox processor core ratio - 1:1
Combined Hub/CAS RAM - 4GB RAM base + 2GB RAM per additional core
Combined Hub/CAS min and max processors – minimum 2 cores and max core is 12 cores
What are the gotchas for combining the two roles?
Load balancing configuration. If you create a CAS array with load balancing what do I do with the Hub on the same server? The answer is you can configure your load balancer to balance inbound SMTP traffic from non-Exchange (Internet, smarthosts, FOPE, etc). The big no-no is to configure Exchange hub to Exchange hub traffic or Hub to Exchange Edge traffic via a VIP/load balancer. The other option is to use DNS round robin for inbound SMTP traffic.
More information sizing the combined CAS/Hub role on TechNet here.