One topic that frequently surfaces for ConfigMgr 2007 R2 Reporting Service is the lack of report models. There are two that ship with the core product. ConfigMgr administrators may want to create additional models depending on need.
Rob Stack (Microsoft) working with the documentation team has created a really nice series of step-by-step guides on this topic:
Step-by-Step Guide to Creating a Report Model in SQL Reporting Services - Simple
http://technet.microsoft.com/en-us/library/cc678428.aspx
Step-by-Step Guide to Creating a Report Model in SQL Reporting Services - Advanced
http://technet.microsoft.com/en-us/library/dd504762.aspx
Step-by-Step Guide to Deploying a Report Model to Configuration Manager
http://technet.microsoft.com/en-us/library/dd504761.aspx