[ruby-core:42285] Why Ruby 1.9 GUI hangs if i do any intensive computation in separate Ruby thread?

Hello

Ruby 1.9 suppose to have native threads, and GIL is supposed to lift
if some threads enters native code (like GUI toolkit main loop or C
implementation of some Ruby lib). But if i start following simple code
sample that displays GUI in main thread and do some basic math in
separate thread - the GUI will hang out badly and dragging window will
redraw ~ every 10 seconds or so :(. I have checked with different GUI
toolkit, Qt (qtbindings gem) - it behaves exactly same. Tested with
Ruby 1.9.3-p0 on Windows 7 and OSX 10.7

require 'tk'
require 'thread'
require 'rexml/document'
Thread.new { loop { a = 1 } }
TkRoot.new.mainloop()

Same code in Python works fine without any GUI hangs:

from Tkinter import *
from threading import *
class WorkThread( Thread ) :
 def run( self ) :
   while True :
     a = 1
WorkThread().start()
Tk().mainloop()

What i'm doing wrong?