I upgraded from 1.0 to 1.2 on Windows XP and when I launch Tomboy it appears in the system tray as usual.

When I click on the icon in the system tray it just disappears.

I launched Tomboy from the command line and this is the output when it crashes:

C:\Program Files\Tomboy>[INFO]: Initializing Mono.Addins
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.IO.FileNotFoundException: Could not load file or assembly 'System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a
5c561934e089' or one of its dependencies. The system cannot find the file specified.
File name: 'System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'
   at Tomboy.TomboyTrayIcon.ShowMenu(Boolean select_first_item)
   at Tomboy.TomboyTrayIcon.OnActivate() in c:\Users\Sandy Armstrong\Desktop\gnome-git\tomboy\Tomboy\Tray.cs:line 176
   at Gtk.StatusIcon.activate_cb(IntPtr status_icon)

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

   at GLib.ExceptionManager.RaiseUnhandledException(Exception e, Boolean is_terminal)
   at Gtk.StatusIcon.activate_cb(IntPtr status_icon)
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Tomboy.WindowsApplication.StartMainLoop() in c:\Users\Sandy Armstrong\Desktop\gnome-git\tomboy\Tomboy\WindowsApplication.cs:li
ne 87
   at Tomboy.Tomboy.StartTrayIcon() in c:\Users\Sandy Armstrong\Desktop\gnome-git\tomboy\Tomboy\Tomboy.cs:line 197
   at Tomboy.Tomboy.Main(String[] args) in c:\Users\Sandy Armstrong\Desktop\gnome-git\tomboy\Tomboy\Tomboy.cs:line 155