After upgrading to libgnome-2.20.1.1 from libgnome-2.20.0 applications
built against the earlier library (glade-2 in this instance) fail with 
an error of the form:

dyld: Library not loaded: /opt/local/lib/libgnome-2.0.dylib
  Referenced from: /opt/local/bin/glade-2
  Reason: Incompatible library version: glade-2 requires version 2001.0.0 or later, but libgnome-2.0.dylib provides version 2000.0.0

inspection of libgnome-2.20.1-2.20.1.1.diff shows:

diff -Nru -x '*.gmo' -x '*.mo' --speed-large-files --minimal libgnome-2.20.1/configure.in libgnome-2.20.1.1/configure.in
--- libgnome-2.20.1/configure.in        2007-10-15 08:42:54.000000000 +0000
+++ libgnome-2.20.1.1/configure.in      2007-10-17 16:44:16.000000000 +0000
@@ -10,10 +10,10 @@
 m4_define([libgnome_major_version], [2])
 m4_define([libgnome_minor_version], [20])
 m4_define([libgnome_micro_version], [1])
-m4_define([libgnome_interface_age], [1])
+m4_define([libgnome_interface_age], [2])
 # If you need a modifier for the version number.
 # Normally empty, but can be used to make "fixup" releases.
-m4_define([libgnome_extraversion], [])
+m4_define([libgnome_extraversion], [.1])

and

diff -Nru -x '*.gmo' -x '*.mo' --speed-large-files --minimal libgnome-2.20.1/configure libgnome-2.20.1.1/configure
--- libgnome-2.20.1/configure   2007-10-15 09:33:49.000000000 +0000
+++ libgnome-2.20.1.1/configure 2007-10-17 16:44:31.000000000 +0000
@@ -2029,13 +2029,13 @@

@@ -2037,11 +2040,11 @@
 LIBGNOME_MAJOR_VERSION=2
 LIBGNOME_MINOR_VERSION=20
 LIBGNOME_MICRO_VERSION=1
-LIBGNOME_INTERFACE_AGE=1
+LIBGNOME_INTERFACE_AGE=2

-LIBGNOME_CURRENT=2000
-LIBGNOME_REVISION=1
-LIBGNOME_AGE=2000
+LIBGNOME_CURRENT=1999
+LIBGNOME_REVISION=2
+LIBGNOME_AGE=1999

from this is appears that:

libgnome_interface_age as defined in libgnome-2.20.1.1/configure.in
has been  incremented to 2 but should remain at 1 since libgnome_micro_version
has not been incremented