The subtypes allocated in g_mime_object_register_type() 
"sub = g_new (struct _subtype_bucket, 1)" aren't freed
in the atexit handling.

Fortunately everything else is cleaned up nicely
on exit, so the fix is just a new line into the
shutdown code you've already done:

--- gmime-object.c.bak  2006-06-23 12:23:41.000000000 -0500
+++ gmime-object.c      2006-06-23 12:24:24.000000000 -0500
@@ -698,6 +698,7 @@
 subtype_bucket_foreach (gpointer key, gpointer value, gpointer user_data)
 {
        g_free (key);
+       g_free (value);
 }