Steps to reproduce:
1. Start f-spot
2. Right click on an image -> Attach tag -> Favorites
3. Right click on anoter image -> Attach tag -> Favorites
4. Crash

Stack trace:
$ f-spot
[Info  00:31:38.640] Initializing DBus
[Info  00:31:38.732] Initializing Mono.Addins
[Info  00:31:38.884] Starting new FSpot server

(f-spot:32466): GdkPixbuf-WARNING **: GdkPixbufLoader finalized without calling gdk_pixbuf_loader_close() - this is not allowed. You must explicitly end the data stream to the loader before dropping the last reference.
Syncing metadata to file...
old = "" new = "" heading = "ASCII"
value = 2008:12:07 11:30:50 len = 19
_:bnode314222720 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag> .
_:bnode314222720 <http://www.w3.org/1999/02/22-rdf-syntax-ns#li> "Favorites" .
<http://fakebase.f-spot.org/internal/> <http://purl.org/dc/elements/1.1/subject> _:bnode314222720 .
value = f-spot version 0.4.4 len = 20
value = 2008:12:24 00:31:45 len = 19
value = f-spot version 0.4.4 len = 20
value = 2008:12:24 00:31:45 len = 19
Saved 10011 bytes
Syncing metadata to file...
old = "" new = "" heading = "ASCII"
value = 2008:12:07 11:07:16 len = 19
_:bnode521728000 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag> .
_:bnode521728000 <http://www.w3.org/1999/02/22-rdf-syntax-ns#li> "Favorites" .
<http://fakebase.f-spot.org/internal/> <http://purl.org/dc/elements/1.1/subject> _:bnode521728000 .
value = f-spot version 0.4.4 len = 20
value = 2008:12:24 00:31:57 len = 19
value = f-spot version 0.4.4 len = 20
value = 2008:12:24 00:31:57 len = 19
Saved 14553 bytes
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
GLib.MissingIntPtrCtorException: GLib.Object subclass FSpot.Widgets.RatingMenuItem must provide a protected or public IntPtr ctor to support wrapping of native object handles.
  at GLib.ObjectManager.CreateObject (IntPtr raw) [0x00000] 
  at GLib.Object.GetObject (IntPtr o, Boolean owned_ref) [0x00000] 
  at Gtk.Container.removed_cb (IntPtr container, IntPtr widget) [0x00000] 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e, Boolean is_terminal)
   at Gtk.Container.removed_cb(IntPtr container, IntPtr widget)
   at GLib.ToggleRef.g_object_remove_toggle_ref(IntPtr , GLib.ToggleNotifyHandler , IntPtr )
   at GLib.ToggleRef.Free()
   at GLib.Object.PerformQueuedUnrefs()
   at GLib.Timeout+TimeoutProxy.Handler()
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Gnome.Program.Run()
   at FSpot.Driver.Main(System.String[] args)



Other information:
Fedora 10 on x86_64. Installed packages:

f-spot-0.4.4-6.fc10.x86_64
mono-core-2.0.1-12.fc10.x86_64
gtk2-2.14.4-3.fc10.x86_64

The crash occurs in many different cases, it is not stable, but f-spot does not survive after just a few mouse clicks. It is not required to attach a tag, just few right and left clicks on image(s) is enough to crash.