In the same vein as my post Watch out, Visual Studio 2010 does not support BI, Visual Studio 2012 does not support BI. Meaning, VS 2012 does not support SSAS, SSIS, or SSRS. If you try to open one of those projects, you will be greeted with:
At least this is better than opening them and converting them to an unusable format, which is what happens when you try to open a VS 2008 BI project in VS 2010.
So, for now, to work with SSAS, SSIS, and SSRS projects you must use SSDT, which uses the Visual Studio 2010 shell. Hopefully, there will be an update to SSDT in the next few months that uses the Visual Studio 2012 shell, without having to wait until the next version of SQL Server. Note you can install SSDT/VS 2010 and VS 2012 on the same computer without any problems.
To see the projects that Visual Studio 2012 supports, check out Visual Studio 2012 RC Compatibility.