SQL Server Data Tools (SSDT) that was released with SQL Server 2012 obviously works with the BI stack (SSIS/SSAS/SSRS) for 2012. But does it also work with the BI stack from SQL Server 2008 R2? Can you use it with 2008 projects and save it as 2008 versions (avoiding having to convert it to 2012)?
SQL Server 2008 engine: Yes! You can use T-SQL (SSMS stuff) and database projects in SSDT against SQL Server 2008 databases
SSRS 2008: Yes! SSDT will convert projects in-place to Visual Studio 2010 (SSDT using the VS 2010 shell) via the Visual Studio Conversion Wizard (so make a copy of it if you want to keep the older version) but you can save any report as a SSRS 2008 version. See Deployment and Version Support in SQL Server Data Tools (SSRS)
SSIS 2008: No! It will convert the project in-place to VS 2010 via the Visual Studio Conversion Wizard and update the SSIS packages via the SSIS Package Upgrade Wizard to SSIS 2012. These packages are permanently upgraded to the format that SQL Server 2012 SSIS uses. See Working with 2008 SSIS packages in 2012 SSDT and Interoperability and Coexistence (Integration Services)
SSAS 2008: No! It will convert the project in-place to VS 2010 via the Visual Studio Conversion Wizard. You can open a SSAS 2008 project and deploy it on SSAS 2012, but you can’t deploy it to SSAS 2008. It will be permanently upgraded to the format that SQL Server 2012 SSAS uses.