This is a known (to some) problem with GDK2.2.4 and graphics tablets,
according to Tor Lilqvist's helpful message:
http://groups.yahoo.com/group/gimpwin-users/message/11945

however, i wanted this information to be reflected here to save
someone else the day of hunting i had to do to workaround this ;-)

on gimp startup:
(gimp-2.0.exe:696): Gdk-WARNING **: losing last reference to
undestroyed window
(gimp-2.0.exe:696): GLib-GObject-CRITICAL **: file gobject.c:
line 1320 (g_object_ref): assertion `object->ref_count > 0' failed

per Tor's advice, as a workaround:
>> Add --ignore-wintab to the command line in the shortcut that you use
>> to start GIMP. Then your tablet's pen should work like a mouse (i.e.,
>> no pressure sensitivity etc).