Another Macports challenging package on Mac OS X 10.6 (and its 64-bit builds) is dia, which fails on building libemf. The recommended solution is to just remove the dependency on libemf, as follows:

--- Portfile.bkup-2009-12-04    2009-09-10 20:16:41.000000000 +1200
+++ Portfile    2009-12-04 15:58:43.000000000 +1300
@@ -39,7 +39,6 @@
     path:lib/pkgconfig/cairo.pc:cairo \
     port:freetype \
     port:libart_lgpl \
-    port:libemf \
     port:libpng \
     port:libxml2 \
     port:libxslt \
@@ -52,6 +51,7 @@

 configure.args \
     --with-cairo \
+    --disable-libemf \
     --with-xslt-prefix=${prefix}

 configure.cppflags-append   -I${prefix}/include/libEMF

Interestingly this seems to be done by default for powerpc:

#
# don't depend on libemf on powerpc platforms
# since it will only build on intel
#

platform powerpc {
    depends_lib-delete port:libemf
    configure.cppflags-delete -I${prefix}/include/libEMF
}

and if we could find a way to match the 64-bit Intel platform then we could use something similar to handle it there. (No one seems interested in updating libemf to be more portable -- presumably doing with ECMA-234 Enhanced MetaFile is easier.)