I just noticed this on the net yesterday.  Microsoft has made available the Active Directory Topology Diagrammer (ADTD).  This tool has been used for a few years now by the MS Premier Active Directory Risk Assessment Teams.  After a risk assessment the customer would be left with this tool and the rights to use it for one year.  I'm glad to see they are releasing it to the public because it's awesome.  The ADTD application discovers your core AD infrastructure and maps your Sites, Site Links, and DCs in a Visio diagram.  You have to see it in action. ADTD should be a requirement if you have to deal with an AD topology of more than 5 sites.  The tool can be download from the Microsoft download site here:

http://www.microsoft.com/downloads/details.aspx?familyid=cb42fc06-50c7-47ed-a65c-862661742764&displaylang=en&tm