I recently came across a posting from someone attending ITForum in Barcelona mentioning that Exchange 2007 has many server roles... "how many servers do we need!"?

This article explains server roles in Exchange 2007 in more depth.

 A "server role" is just a logical grouping of functionality in Exchange.  In the past, when you installed Exchange, you got it ALL -- and then you had to turn on or off particular features later to determine how that server would function.  (Would it be primarily a mailbox server, a transport server, a front-end server, etc.)? 

 Server roles enable you to declare ahead of time how you are going to use that particular Exchange instance in your enviornment.  It means that after install, you have to do much less of that customization to special-purpose your server.  For example, if you are going to use this server primarily as a client access server (for OWA), you probably don't need a mailbox database on it.  In Exchange 2007, we take care of that for you if you select only the client access role.  It also means that when you are using the Exchange Management Console you can easily tell how you are using that particular server in your environment. 

 However, 'server roles' do NOT mean that you have to use more physical machines in your deployment.  It is possible to put all server roles (with the exception of Edge) on the same computer.