Many people think that Windows SBS networks can only have one Domain Controller. Let me set the record straight right now.
All versions of Small Business Server can have any number of additional servers, and even additional domain controllers. SBS only has to hold the FSMO roles. With the new SBS 2008 Premium edition, the second server can be used as a domain controller, or you can add any number of servers. The Premium edition of SBS 2008 is just another copy of Windows Server, a cheaper version, to get you started on your server farm.
All the copies of SBS, you can add any number of servers that you want!