How do you switch a SharePoint farm / site collection from NTLM to Kerberos?
It seems that there more to it than selecting Kerberos in:
Central Administration > Application Management > Authentication Providers > Edit Authentication
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Kerberos is actually configured at the web application level. So you do have the option of setting it for your primary site collection, for example, but not for central administration if you wanted to.
Here is a link to get you started. There is actually a lot to it. http://technet.microsoft.com/en-us/library/cc263449(office.12).aspx
My recommendation would be to create a new web application with its own site collection specifically for testing your kerberos set up. Set the switch you mention above on the new web app only when you are ready to test. The reason I say that is because if you set it on your Central Admin or production web app and it doesn't work, then you may not be able to authenticate to get back in and change it back.