http://blogs.technet.com/sbs/a...lica-dcs-in-an-sbs-domain.aspx
- There can be only one Small Business Server in the domain. (You can actually have more than one Small Business Server in the same domain for up to 7 days before one or both of them start shutting down every hour. This can be useful in some migration scenarios. However be sure to finish the migration and then demote and remove one of them from the network before they start shutting down!)
- The Small Business Server must be installed into the root domain of a single-domain forest.
- SBS does not support trusts with other domains or forests, including both implicit and explicit trusts.
- SBS does not support parent or child domains.
- The Small Business Server must own all 5 of the forest and domain FSMO roles.
- The Small Business Server must be a global catalog server.
- SBS supports up to 75 user or device CALs.
- The components that come with SBS such as Exchange, SQL, and ISA can only be installed on the server that is running SBS.
- Additional servers can be joined to or promoted to the domain but separate server licenses must be obtained for each additional server. However expanded client access license (CAL) rights in Windows SBS 2003 R2 allow client computers to access additional servers on the network (such as those running Exchange Server 2003, SQL Server 2005 Workgroup Edition, and Windows Server) without requiring additional CALs. This applies to additional DCs as well.