Hey Russ,

Are you using shared data sources? Make sure the connection string has the correct servername and that from the SSRS server you can telnet into the DB server. This should resolve at least any networking issues. Then I would say make sure whatever account you are using for your data source credentials has access to the database(s) where your query/stored proc pulls from. This needs to be done through SQL Server Management Studio.

Also, are the SSRS and DB servers on the same domain? That might cause some problems as well...