By the way I branched the Addins and the master branch now depends on F-Spot’s core 0.7, which replaces Core with App.Instance.
Update: The tabbed sidebar is not working at the moment with 0.7 (git), due to a redesign in the f-spot source. I am still not sure how I redesign the tabbed sidebar code to get it work with the actual design or if the f-spot code will be changed again. 0.6 works fine.
See my F-Spot Addin page for more information.