Yesterday the Windows Server team released an important hotfix for anyone using static routes on a Windows Server 2008 or Windows Server 2008 R2 cluster. The issue is that manually added routes, like static routes that many of us use, are deleted from the route table when you delete an IP address from the network adapter. This is exactly what happens when a cluster IP address resource moves from one node to another.
If you use Windows Server 2008 or Windows Server 2008 R2 and static routes, you should read this article and determine if you need to apply this hotfix to your servers.
Note: the hotfix also applies to Windows Vista and Windows 7
Hotfix: Manually added route table entries are deleted unexpectedly when you delete an additional IP address in Windows Vista, in Windows 7, in Windows Server 2008 or in Windows Server 2008 R2