Tip - How does Windows Server 2008 resolve Domain Controller Load Balancing problems?
This article discusses a new Group Policy setting which solves the Load Balancing issues with the domain controllers.
The DC Locator Service has been re-designed in Windows Server 2008 to include a new mechanism. When a client computer finds a preferred domain controller, it sticks to this domain controller unless that domain controller stops responding or the client computer is restarted. This is generally called Domain Controller Stickiness.
Read more here...