This has been brought up on ros answers a while ago, but this minor bug is still around in Fuerte (on Ubuntu Precise 64bit).

http://answers.ros.org/question/10777/service-exception-using-tf-listener-in-rospy/

And regarding Tully's answer, he is right, this exception is only thrown on creation.

PS: I hope this is the right place for new tickets or should I report this on trac?
PPS: If this is the right place, we should update the "rospy wiki":http://www.ros.org/wiki/rospy.