patch

diff --git a/vm/builtin/thread.cpp b/vm/builtin/thread.cpp
index 314ffda..d772123 100644
--- a/vm/builtin/thread.cpp
+++ b/vm/builtin/thread.cpp
@@ -256,6 +256,7 @@ namespace rubinius {
 
     VM* vm = vm_;
     if(!vm) return nil<Array>();
+    StopTheWorld stop(vm);
 
     CallFrame* cf = vm->saved_call_frame()->top_ruby_frame();

wtf.rb

t = Thread.start do
  sleep 1
end

p t.backtrace
sleep 2
p t.backtrace