Recently an upgrade from SSRS 2005 to 2008 threw our team for a loop.  The exact connection in 2005 was working perfectly, but in 2008, the reports were failing.  What was the issue, and how did we fix it?

For expression-based connection strings that reference report parameters, make sure that these parameters are at the beginning of the list of report parameters and don't depend on other report parameter values that may not be evaluated yet (e.g. if there are hierarchical parameters and the parameter used in the connection string depends on data from an earlier parameter with dataset-dependent valid values / default values of the data source it is about to connect to - that won't work).

All that was needed was to make sure the @DB parameter we used was at the top of the list >> tah-dah....it worked!

Special thanks to Robert Bruckner http://blogs.msdn.com/robertbruckner for posting this approach.