Here is the full assertion:
(gnome-theme-thumbnailer:10608): Gdk-CRITICAL **: gdk_drawable_set_colormap:
assertion `cmap == NULL || gdk_drawable_get_depth (drawable) ==
cmap->visual->depth' failed

The assertion is from line ~180 of capplets/common/theme-thumbnail.c:
static void
create_image (ThemeThumbnailData *theme_thumbnail_data,
	      GdkPixbuf          *pixbuf)
{
...
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
...
  /* Create a pixmap */
  visual = gtk_widget_get_visual (window);
  pixmap = gdk_pixmap_new (NULL, ICON_SIZE_WIDTH, ICON_SIZE_HEIGHT,
gdk_visual_get_best_depth());
  gdk_drawable_set_colormap (GDK_DRAWABLE (pixmap), gtk_widget_get_colormap
(window));


The problem is the pixmap is created with the depth from
gdk_visual_get_best_depth(). Then set colormap tries to use the colormap of a
window with a different depth. That is the pixmap has depth 32 and the window
depth 24. This may have to do with certain versions of xorg and/or nvidia
drivers. I will attach a patch witch uses the depth of the window's visual.