After creating a 2012 AddIn project using the wizard, and comparing to my old 2011 AddIn project, I managed to update my 2011 project using the following steps:
1 remove all COM registration
2 create a .addin file in the c:\ProgramData\Autodesk\Inventor 2012\AddIns folder
(this file will tell Inventor where your AddIn dll resides, replaces the COM registration method)
3 make sure your project references the new Inventor Interop DLL version 16...
4 add the manifest build command to your post build events
(call "%VS90COMNTOOLS%vsvars32" mt.exe -manifest "$(ProjectDir)ExactFlatInventor2012.X.manifest" - outputresource:"$(TargetPath)";#2)
I'm not sure why or if this is needed, but the wizard created this so I used it.
5 Your installer must place the .addin file in the programdata folder, and refer to the proper path regardless of where the AddIn is installed. I created a custom installer class to do this. You could also install to the Inventor bin directory I think, and Inventor will automatically find it.
I hope I'm not missing anything, please let me know if I am.