Recently, the "Report Menu" stopped appearing in the Main Menu in Visual Studio 2005 after focusing on the report designer surface (in an ASP.NET project). This is a rather significant problem for me, since I do not know of another way to modify report parameters and data sources....
I did a bit of digging through the activity log (as described in Troubleshooting Extensions with the Activity Log on the Visual Studio Blog, but found that the Microsoft Report Designer Package was loading without error. The real breakthrough came after watching devenv.exe in Process Monitor and watching it load the Visual Studio 2008 versions of Microsoft.ReportDesignerUI.dll and several others....
My guess is that the cause of the problem was installing Business Intelligence Development Studio with the "Advanced Services" skew of SQL Server 2008 R2 Express, which uses Visual Studio 2008 and a version of the Microsoft Report Designer Package designed for that version of VS. However, I have not confirmed this absolutely, because I need the newer version for another project. So, instead, I will bite the bullet and upgrade all of my Report Viewer 2005 reports to Report Viewer 2008. At least then I can edit them in Visual Studio 2010 (did I mention Report Designer 2008 - in VS 2010 - won't edit/save 2005 files?).
In case this sounds like a familiar gripe, I had similar problems with incompatible library versions in Microsoft Access.