Microsoft Knowledge Base Article 306602 contains the following summary:
The domain controller locator mechanism in Windows 2000
always prefers a domain controller that resides in the site of the client that
is searching for a domain controller, which is achieved by a domain controller
that registers site-specific domain controller locator DNS SRV resource records
for the site in which the domain controller resides.
In addition, a domain controller may register site-specific domain controller locator DNS SRV resource records for any other sites that do not contain a domain controller in the same role (such as one that hosts the same domain, or that is a Global Catalog) to which the site of the domain controller is the closest. This ensures that clients locate the nearest domain controller in case no domain controller is located in the client's site.
For more information about this mechanism, refer to the Windows 2000 Server Resource Kit, "Distributed Systems Guide" book, Chapter 3 "Name Resolution in Active Directory".
In a case in which all the domain controllers in the same role (hosting the same domain, or being Global Catalogs) in a particular site become unavailable, clients that are located in the same site will fail over to any other domain controller in any other site with no optimization.