In child_program JRuby (tested with 1.1) throws SystemExit which contains the exit status.

systemu.rb
Index: .
===================================================================
--- .	(revision 602)
+++ .	(working copy)
@@ -188,6 +188,9 @@
         PIPE.close
 
         exec *argv
+      rescue SystemExit => e
+        # jruby
+ 	    exit e.status       
       rescue Exception => e
         PIPE.write Marshal.dump(e) rescue nil
         exit 42