opengl window background transparent

I am new to opengl and I am making an opengles 2.0 application where i need to make
the background of the window transparent and then show a blended object
over it. Please can someone tell me how can i make window background
transparent. The appearance i want is that it sud appear as if only a
blended object is in front.
Any help will be greatly appreciated. Thanks in advance.
With regards,
aditya.sen2009-11-27 12:56:12