I received a call from one of my colleagues in the Oakton Canberra office today who was working at a Federal Government department. He was using a SharePoint-integrated mode of SSRS.
He wanted deployed reports to point to the correct datasource without manually updating the DataSource of the report each time a new site is provisioned. It was not a feasible option for him to be manually changing the report datasources each time a new site in SharePoint was deployed (based on a particular site template that included Reports) - especially when he moved back off site.
I suggested a couple of ways forward - each with their own advantages and disadvantages:
1) Create a custom SQL Reporting Services Reports Extension that can source data dynamically (as outlined here http://msdn.microsoft.com/en-us/library/microsoft.reportingservices.dataprocessing.aspx and here http://msdn.microsoft.com/en-us/library/bb283184.aspx)
2) Using Expression Based Connection Strings
3) Setting the datasource at Deploy Time instead via Powershell or another scripting technology (or as part of a feature event receiver in SharePoint)
The Deploy-time option was the simplest and cleanest option (though it would have a limitation of breaking if the site locations are moved). A script would need to update the datasources as part of the site migration process. This was deemed to be an acceptable tradeoff.