The build system tries to link to installed system libraries first, which 
fails because the ABI has changed...

diff -ruN mlt-0.7.2.orig/src/swig/ruby/build mlt-0.7.2/src/swig/ruby/build
--- mlt-0.7.2.orig/src/swig/ruby/build	2011-05-02 07:59:12.000000000 +0200
+++ mlt-0.7.2/src/swig/ruby/build	2011-10-30 16:21:01.000000000 +0100
@@ -5,8 +5,10 @@
   exit 0
 end
 system( "ln -sf ../mlt.i" )
+system( "ln -sf ../../framework/libmlt.so" )
+system( "ln -sf ../../mlt++/libmlt++.so" )
 system( "swig -c++ -ruby -I../../mlt++ -I../.. mlt.i" )
 $CFLAGS += " -I../.."
-$LDFLAGS += " -L../../mlt++ -lmlt++"
+$LIBS += " -lmlt++ -lmlt -lstdc++"
 create_makefile('mlt')
 system( "make" )