I've actually investigated this quite a bit further, but haven't
had the chance to write it up.  Instead of the original patch, I have
been running more recently with the patch included below to the guile
buildlink support, which adds this for all guile dependent packages,
but it's basically the same idea.

I've tracked this down much further and determed that on most
platforms, the ltdl will eventually fall back to the system dlopen
which will look in the rpath coded in the binary.  However, darwin
doesn't have an rpath, and instead uses abolute paths on each of the
libraries listed in a binary.  (There is a tools for editing them if
they move, install_name_tool(1)).  Therefore darwin requires the
environment var to be set for guile to find its modules, since
loaded modules do not have an abolute path in the binary.

Btw, guile doesn't need a buildlink or dependency on devel/libltdl,
it ships with its own modified copy of ltdl which is the one it
seems to be using.

Thanks,
Darrin

Index: buildlink3.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/guile/buildlink3.mk,v
retrieving revision 1.11
diff -u -r1.11 buildlink3.mk
--- buildlink3.mk       8 Jul 2006 23:10:55 -0000       1.11
+++ buildlink3.mk       7 Oct 2006 21:53:59 -0000
@@ -17,7 +17,12 @@
 BUILDLINK_PKGSRCDIR.guile?=    ../../lang/guile
 .endif # GUILE_BUILDLINK3_MK
 
+.include "../../mk/bsd.prefs.mk"
+.if ${OPSYS} == "Darwin"
+CONFIGURE_ENV+=                LTDL_LIBRARY_PATH=${LOCALBASE}/lib
+MAKE_ENV+=             LTDL_LIBRARY_PATH=${LOCALBASE}/lib
+.endif
+
 .include "../../devel/libltdl/buildlink3.mk"
 .include "../../devel/ncurses/buildlink3.mk"
 .include "../../devel/readline/buildlink3.mk"
 .include "../../mk/pthread.buildlink3.mk"