This post is more about the confusion that may arise around SPNs for setting up Kerberos authentication in IIS 7.0. IIS 7.0 has a new Kernel-mode authentication feature using which the ticket for the requested service is decrypted using Machine account (Local system) of the IIS server. It no longer depends upon the application pool Identity for this purpose by default and in turn improves the performance.
The are good post SharePoint Farm using Kerberos on IIS7 related to this configuration.
In Windows Server 2008, kernel mode authentication runs under the machine account, but Office Communications Server 2007 R2 runs under a user account. As a result, Kerberos service ticket decryption fails if kernel mode authentication is enabled. If you install and activate Office Communications Server 2007 R2 on a computer running the Windows Server 2008 operating system, Setup disables kernel mode authentication in IIS to support Kerberos.
This step-by-step article describes how to configure Microsoft Internet Information Services (IIS) to support both the Kerberos protocol and the NTLM protocol for network authentication.