Quoting http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=474666 :

Tasque refuse to work on my machine here is the output:

Debug]: Tasque remote control active.
[Debug]: Found Available Backend: Tasque.Backends.RtmBackend.RtmBackend
[Debug]: Found Available Backend: Tasque.Backends.Sqlite.SqliteBackend
[Debug]: Found Available Backend: Tasque.Backends.EDS.EDSBackend
[Debug]: Tasque.exe location:  /usr/lib/tasque/Tasque.exe
[Info]: Searching for Backend DLLs in: /usr/lib/tasque
[Info]: 	Reading /usr/lib/tasque/RtmNet.dll
[Debug]: Storing 'Tasque.Backends.RtmBackend.RtmBackend' = 'Remember the Milk'
[Debug]: Storing 'Tasque.Backends.Sqlite.SqliteBackend' = 'Local File'
[Debug]: Storing 'Tasque.Backends.EDS.EDSBackend' = 'Evolution Data Server'
[Debug]: CurrentBackend specified in Preferences: 
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.DllNotFoundException: gdk-x11-2.0
  at (wrapper managed-to-native) Egg.TrayIcon:gdk_x11_display_get_xdisplay (intptr)
  at Egg.TrayIcon.OnRealized () [0x00000] 
  at Gtk.Widget.realized_cb (IntPtr widget) [0x00000] 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e, Boolean is_terminal)
   at Gtk.Widget.realized_cb(IntPtr widget)
   at Gtk.Widget.realized_cb(IntPtr )
   at Gtk.Widget.gtk_widget_show_all(IntPtr )
   at Gtk.Widget.gtk_widget_show_all(IntPtr )
   at Gtk.Widget.ShowAll()
   at Tasque.Application.SetupTrayIcon()
   at Tasque.Application.InitializeIdle()
   at GLib.Idle+IdleProxy.Handler()
   at GLib.Idle+IdleProxy.Handler()
   at Gtk.Application.gtk_main()
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Gnome.Program.Run()
   at Tasque.Application.StartMainLoop()
   at Tasque.Application.Main(System.String[] args)
[1]    9212 segmentation fault  tasque

Installing libgtk2.0-dev fixed the problem although I don't know how.

==================

I'll fix this in SVN later, trivial change to the DllImports.