On a fresh install of Ubuntu 11.10 (running Unity) added daily ppa for the Amazon .amz fix. Whenever I click "Amazon MP3 Store", Banshee immediately crashes with the following error in the terminal:

Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.NullReferenceException: Object reference not set to an instance of an object
  at Banshee.WebSource.WebSource.Activate () [0x00000] in <filename unknown>:0 
  at Banshee.Sources.SourceManager.SetActiveSource (Banshee.Sources.Source source, Boolean notify) [0x00000] in <filename unknown>:0 
  at Banshee.Sources.SourceManager.SetActiveSource (Banshee.Sources.Source source) [0x00000] in <filename unknown>:0 
  at Banshee.Sources.Gui.SourceView.OnButtonPressEvent (Gdk.EventButton press) [0x00000] in <filename unknown>:0 
  at Gtk.Widget.buttonpressevent_cb (IntPtr widget, IntPtr evnt) [0x00000] in <filename unknown>:0 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e, Boolean is_terminal)
   at Gtk.Widget.buttonpressevent_cb(IntPtr widget, IntPtr evnt)
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Banshee.Gui.GtkBaseClient.Run()
   at Banshee.Gui.GtkBaseClient.Startup()
   at Hyena.Gui.CleanRoomStartup.Startup(Hyena.Gui.StartupInvocationHandler startup)
   at Banshee.Gui.GtkBaseClient.Startup()
   at Banshee.Gui.GtkBaseClient.Startup(System.String[] args)
   at Nereid.Client.Main(System.String[] args)
   at System.AppDomain.ExecuteAssembly(System.AppDomain , System.Reflection.Assembly , System.String[] )
   at System.AppDomain.ExecuteAssemblyInternal(System.Reflection.Assembly a, System.String[] args)
   at System.AppDomain.ExecuteAssembly(System.String assemblyFile, System.Security.Policy.Evidence assemblySecurity, System.String[] args)
   at System.AppDomain.ExecuteAssembly(System.String assemblyFile)
   at Booter.Booter.BootClient(System.String clientName)
   at Booter.Booter.Main()

If you need more information, let me know and I'll cheerfully try and get it for you.