The official way to detect path to Mono runtime is to run pkg-config.
The following patch should resolve TODO.
Index: Rakefile
==================================================================--- Rakefile  
(revision 53)
+++ Rakefile    (working copy)
@@ -32,7 +32,8 @@
 if ENV[''mono''].nil?
   FRAMEWORK_DIR = Pathname.new(ENV[''windir''].dup) +
''Microsoft.NET'' +
''Framework'' + ''v2.0.50727''
 else
-  FRAMEWORK_DIR = Pathname.new(''/usr/local/lib/mono/2.0'')  #
TODO:
get mono path from ENV
+  libdir = IO.popen(''pkg-config --variable=libdir
mono'').read.strip
+  FRAMEWORK_DIR = Pathname.new(libdir) + ''mono'' +
''2.0''
 end
 CS_COMPILER             = ENV[''mono''].nil? ?
''csc'' : ''gmcs''
-- 
Seo Sanghyeon