I ran across a document from Microsoft that lists maximum limits for Active Directory. This document pertains to Windows 2000 Server and Windows Server 2003. There is no reference to Windows Server 2008 in the document. However, the majority of the limits also apply to Windows Server 2008. Below is a summary of the maximums. The full details, including rationale, can be found here: http://technet.microsoft.com/en-us/library/cc756101.aspx.