Adam Saxton posted his great Kerberos troubleshooting guide, which covers both Reporting Services in native mode as well as SharePoint integrated mode configurations:
If you are specifically troubleshooting Kerberos with a Reporting Services 2008 deployment, you may also want to check out the following whitepaper:
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.