How to: Register a Service Principal Name (SPN) for a Report Server - NTLM authentication - Kerberos

http://msdn.microsoft.com/en-us/library/cc281382.aspx

When Kerberos authentication is required

Kerberos is a network authentication protocol that allows Windows integrated authentication to occur across multiple computers (a client and multiple servers). NTLM authentication is an alternative to Kerberos authentication, and it is based on a challenge-response mechanism between a client and a server. NTLM authentication is easier to configure than Kerberos; however, it supports the flow of credentials between just two computers (a client and one server).
Kerberos authentication must be used in Microsoft Dynamics AX environments where: Enterprise Portal is installed on a server other than the server running Reporting Services and/or Analysis Services.
The Reporting Services database exists on a server other than the server running the Reporting Services Windows service.
If you do not configure Kerberos authentication, users will not be able to view Reporting Services reports and/or Analysis Services reports in Role Center pages.