JRuby 1.6.4 incompatible with Google App Engine

It appears that JRuby 1.6.4 introduces some incompatibilities with
AppEngine. The following error occurs- something to do with support
for Scala? The only workaround is to downgrade jruby-jars to 1.6.3.

Brief discussion at: http://groups.google.com/group/appengine-jruby/browse_thread/thread/9dae8ddbe1d1a403

javax.servlet.ServletContext log: Application Error
org.jruby.exceptions.RaiseException: Native Exception: 'class
java.security.AccessControlException'; Message: access denied
(java.lang.RuntimePermission getClassLoader); StackTrace:
java.security.AccessControlException: access denied
(java.lang.RuntimePermission getClassLoader)
at com.google.appengine.runtime.Request.process-cf6eefcb9b61eeaa(Request.java)
at java.lang.Class.getClassLoader(Class.java:437)
at org.jruby.javasupport.JavaClass.handleScalaSingletons(JavaClass.java:130)
at org.jruby.javasupport.JavaClass.setupClassMethods(JavaClass.java:1006)
at org.jruby.javasupport.JavaClass.access$700(JavaClass.java:99)
at org.jruby.javasupport.JavaClass$ClassInitializer.initialize(JavaClass.java:650)
at org.jruby.javasupport.JavaClass.setupProxy(JavaClass.java:689)
at org.jruby.javasupport.Java.createProxyClass(Java.java:520)
at org.jruby.javasupport.Java.getProxyClass(Java.java:449)
at org.jruby.javasupport.Java.getInstance(Java.java:358)
at org.jruby.javasupport.JavaUtil.convertJavaToUsableRubyObject(JavaUtil.java:165)
at org.jruby.javasupport.JavaMethod.convertReturn(JavaMethod.java:595)
at org.jruby.javasupport.JavaMethod.invokeDirectWithExceptionHandling(JavaMethod.java:528)
at org.jruby.javasupport.JavaMethod.invokeDirect(JavaMethod.java:380)
at org.jruby.java.invokers.InstanceMethodInvoker.call(InstanceMethodInvoker.java:59)
at org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:167)
at org.jruby.ast.CallOneArgNode.interpret(CallOneArgNode.java:57)
at org.jruby.ast.DAsgnNode.interpret(DAsgnNode.java:110)
at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:104)
at org.jruby.ast.BlockNode.interpret(BlockNode.java:71)