SBS 2011 Essentials is a bit different from all other Microsoft servers in the past. Traditional network design calls for the Domain Controllers to have a FIXED IP address within your network. Well in SBS 2011 Essentials, Microsoft understood that most clients could not figure out how to disable the DHCP on their routers and therefore they redesigned SBS 2011 Essentials to work with a DHCP assigned IP address.
This creates unique problems however for the domain joined computers as they need to point to the DNS for the Domain Controller to work correctly. Microsoft’s John Bay explains more about how this works here in todays blog post on the SBS support blog. Hope this helps explain it.