Steps to reproduce:
1. Load a VS2012 project/solution that uses Qt.
2. Open Project Properties.
3. Go to C++->General page.
4. Make a change to Additional Include Directories.
5. Click Apply. ---> hang
If the Qt VS Add-in is enabled, the VS2012 GUI will hang for a long time (a few minutes to ~1hr), depending on the size of the project. If you wait long enough, the operation finishes and you can resume using the GUI.
If the add-in is disabled (e.g. via the menu Tools->Add-in Mannager...), then there is no hang.