Adam Saxton posted his great Kerberos troubleshooting guide, which covers both Reporting Services in native mode as well as SharePoint integrated mode configurations:

http://blogs.msdn.com/b/psssql/archive/2010/06/23/my-kerberos-checklist.aspx

If you are specifically troubleshooting Kerberos with a Reporting Services 2008 deployment, you may also want to check out the following whitepaper:

http://msdn.microsoft.com/en-us/library/ff679930.aspx

Update (July 18):

A new whitepaper for Configuring Kerberos Authentication for SharePoint 2010 products has just been released.  Section 4 of that whitepaper explains identity delegation for Reporting Services.

http://go.microsoft.com/fwlink/?LinkID=196600