“Package load failure”. Package ‘VSCorePackage’ has failed to load properly ( GUID={7494682B-37A0-11D2-A273-00C04F8EF4FF} ). Please contact package vendor for assistance.  

Uninstalled, rebooted, reinstalled, rebooted, problem sticks.  :-(

Update: Apparently this error isn't really a sign of complete failure of the development environment, but rather a result of project file corruption. Before the problem occured, the C# compiler in VS tanked and obviously trashed the project file.  New projects still work as expected, but the project that causes this failure is broken.