[ros-users] rospy.sleep() blocking callbacks

I'm writing on a GUI in Python/Tk - I have a subscriber which is
listening for image messages and setting a class member variable with
the newest image, and a GUI element that needs to use image (after
sending a message to move a pan/tilt). On click I use rospy.sleep() to
wait for the pan/tilt to settle down before using any images. However,
I've noticed that once I call rospy.sleep() inside the click callback,
the callback for the image subscriber stops getting called. It was my
understanding that subscriber callbacks are run asynchronously in
their own thread, so a sleep in a different thread should have no
effect. What's going on here? What am I missing?

Thanks,
-Dan